Python Bootcamp · Run the Python interpreter interactively, so you can type code right at it........
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Yanqiao ZHU https://sxkdz.org Tongji Apple Club School of Software Engineering, Tongji University . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Python Bootcamp Apple Workshop (Spring 2018)
Transcript of Python Bootcamp · Run the Python interpreter interactively, so you can type code right at it........
Yanqiao ZHUhttps://sxkdz.orgTongji Apple ClubSchool of Software Engineering, Tongji University
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Python BootcampApple Workshop
(Spring 2018)
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Outline
1 Getting StartedSetting Up EnvironmentEditing Python
2 Introduction to PythonLanguage IntroductionFeatures
3 Basic SyntaxFirst Python ProgramPython IdentifiersLines and IndentationCommentsCommand-Line ArgumentsCode Checked at Runtime
4 Data Types and OperatorsVariablesBuilt-in TypesBasic Operators
5 Control StatementsBranchesLoops
6 Functions7 Modules8 Exceptions9 Closing Up and Further More
Getting HelpBuilt-in Documentation
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Getting Started
Python Set Up
Python is free and open source, available for all operating systemsfrom python.org. In particular we want a Python install where youcan do two things:▶ Run an existing python program, such as hello.py.▶ Run the Python interpreter interactively, so you can type code
right at it.
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Getting Started
Python on Linux and macOS
Most operating systems other than Windows already have Pythoninstalled by default. To check that Python is installed, open theterminal and type python in the terminal to run the Pythoninterpreter interactively:
sxkdz:~$ pythonPython 3.6.5 (default, Mar 30 2018, 06:42:10)[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang
-900.0.39.2)] on darwinType "help", "copyright", "credits" or "license"
for more information.>>> print('Hello, world')Hello, world
Listing 1.1: First Run
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Getting Started
Python on Windows
1. Go to the download page, select a version (the legacy versionis 2.x, the newest version is 3.6.5).
2. Run the Python installer, taking all the defaults. This willinstall Python in the root directory and set up some fileassociations.
To run the Python interpreter interactively, select the Run…command from the Start menu, and type python – this will launchPython interactively in its own window.
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Getting Started
Exit Interactive-Mode Python
On Windows, use Ctrl + Z to represent EOF (End-of-File) andexit.
On all other operating systems, EOF is represented by + D orCtrl + D .
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Getting Started
Editing Python
A Python program is just a text file that you edit directly. Asabove, you should have a command line open, where you can typepython hello.py Alice to run whatever you are working on. Atthe command line prompt, just hit the key to recall previouslytyped commands without retyping them.
You want a text editor with a little understanding of code andindentation. There are many good free ones:
1. Notepad++ (free and open source, Windows only)2. Visual Studio Code with Python or MagicPython extension
(free and open source, cross platform)3. PyCharm (professional, cross platform, free for students)4. Thonny (official recommended, free, cross platform)
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Introduction to Python
Language Introduction
Python is a clear and powerful object-oriented programminglanguage, comparable to Perl, Ruby, Scheme, or Java.
Python is a dynamic, interpreted (bytecode-compiled) language.There are no type declarations of variables, parameters, functions,or methods in source code. This makes the code short and flexible,and you lose the compile-time type checking of the source code.Python tracks the types of all values at runtime and flags codethat does not make sense as it runs.
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Introduction to Python
Notable Features
✓ Easy-to-learn: has an elegant syntax, few keywords andsomple structure.
✓ Easy-to-read: Python code is clearly defined and visible to theeyes.
✓ Ideal for prototype development and other ad-hocprogramming tasks, without compromising maintainability.
✓ A board standard library: supports many commonprogramming tasks such as connecting to web servers,searching text with regular expressions, reading and modifyingfiles.
✓ Extendable: can add new modules implemented in a compiledlanguage such as C or C++.
✓ Cross-platform compatible.
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Introduction to Python
Features at Programming-Language Level
✓ A variety of basic data types are available.✓ Python supports object-oriented programming with classes
and multiple inheritance.✓ Code can be grouped into modules and packages.✓ The language supports raising and catching exceptions,
resulting in cleaner error handling.
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Introduction to Python
Features at Programming-Language Level (cont.)
✓ Data types are strongly and dynamically typed. Mixingincompatible types (e.g. attempting to add a string and anumber) causes an exception to be raised, so errors arecaught sooner.
✓ Python contains advanced programming features such asgenerators and list comprehensions.
✓ Python’s automatic memory management frees you fromhaving to manually allocate and free memory in your code.
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Introduction to Python
The Zen of Python
Python aficionados are often quick to point out how “intuitive”,“beautiful”, or “fun” Python is.
A nice little Easter egg exists in the Python interpreter–simplyclose your eyes, meditate for a few minutes, and run importthis, you will see the zen of Python, by Tim Peters.
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Basic Syntax
First Python Program
There’re two ways of programming in Python:
Interactive Mode ProgrammingInvoking the interpreter without passing a script file as a parameterbrings up the prompt like Listing 1.1.
Script Mode ProgrammingInvoking the interpreter with a script parameter begins executionof the script and continues until the script is finished. When thescript is finished, the interpreter is no longer active.
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Basic Syntax
First Python Program (cont.)
Let us write a simple Python program in a script. Python files haveextension .py. Type the following source code in a hello.py file:
1 #!/usr/bin/python323 print("Hello, Python!")
Listing 3.1: First Python Program
We assume that you have Python interpreter available in/usr/bin directory.
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Basic Syntax
First Python Program (cont.)
Now, try to run this program as follows:
$ chmod +x hello.py$ ./hello.py
Listing 3.2: Modify the Property of the Script and Run
This produces the following result:
Hello, Python!
Listing 3.3: Result of the Script
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Basic Syntax
First Python Program (cont.)
ShebangThe first line in Listing 3.1 is called Shebang.Under Unix-like operating systems, when a script with a shebang isrun as a program, the program loader parses the rest of the script’sinitial line as an interpreter directive; the specified interpreterprogram is run instead, passing to it as an argument the path thatwas initially used when attempting to run the script.A valid shebang begins with number sign and exclamation mark(#!) and is at the beginning of a script.
chmod in Listing 3.2 is to make the script file executable.
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Basic Syntax
Python Identifiers
A Python identifier is a name used to identify a variable, function,class, module or other object. An identifier starts with a letter A toZ or a to z or an underscore (_) followed by zero or more letters,underscores and digits (0 to 9).
Python does not allow punctuation characters such as @, $, and %within identifiers. Python is a case sensitive programminglanguage. Thus, Foobar and foobar are two different identifiers inPython.
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Basic Syntax
Python Identifiers (cont.)
Here are naming conventions for Python identifiers:▶ Class names start with an uppercase letter. All other
identifiers start with a lowercase letter.▶ Starting an identifier with a single leading underscore
indicates that the identifier is private.▶ Starting an identifier with two leading underscores indicates a
strongly private identifier.▶ If the identifier also ends with two trailing underscores, the
identifier is a language-defined special name.
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Basic Syntax
Variable Names
Since Python variables don’t have any type spelled out in thesource code, it’s extra helpful to give meaningful names to yourvariables to remind yourself of what’s going on. So use:▶ name – if it’s a single name▶ names – if it’s a list of names▶ tuples – if it’s a list of tuples
In general, Python prefers the underscore method but guidesdevelopers to defer to camelCasing if integrating into existingPython code that already uses that style. Readability counts.
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Basic Syntax
Reserved Words
Table 3.1 shows the Python keywords. These are reserved wordsand can’t be used as constant or variable or any other identifiernames. All the Python keywords contain lowercase letters only.
auto del for is raiseassert elif from lambda returnbreak else global not tryclass except if or while
continue exec import pass withdef finally in print yield
Table 3.1: Python Reserved Words
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Basic Syntax
Lines and Indentation
Python provides no braces to indicate blocks of code for class andfunction definitions or flow control. Blocks of code are denoted byline indentation, which is rigidly enforced. A logical block ofstatements such as the ones that make up a function should allhave the same indentation, set in from the indentation of theirparent function or if or whatever. If one of the lines in a grouphas a different indentation, it is flagged as a syntax error.
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Basic Syntax
Lines and Indentation (cont.)
Avoid using TAB s as they greatly complicate the indentationscheme.
A common question beginners ask is, “How many spaces should Iindent?” According to the official Python style guide (PEP 8), youshould indent with 4 spaces. (Fact: Google’s internal styleguideline dictates indenting by 2 spaces.)
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Basic Syntax
Multi-Line Statements
Statements in Python typically end with a new line. Python does,however, allow the use of the line continuation character (\) todenote that the line should continue. For example:
1 foo = bar_one + \2 bar_two + \3 bar_three
Listing 3.4: A Multi-line Statement
Statements contained within the [], {}, or () brackets do notneed to use the line continuation character.
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Basic Syntax
Using Blank Lines
A line containing only whitespace, possibly with a comment, isknown as a blank line and Python totally ignores it.
In an interactive interpreter session, you must enter an emptyphysical line to terminate a multi-line statement.
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Basic Syntax
Multiple Statements on a Single Line
The semicolon (;) allows multiple statements on the single linegiven that neither statement starts a new code block. Here is asample snip using the semicolon:
1 import sys; x = 'foo'; sys.stdout.write(x + '\n')
Listing 3.5: Multiple Statements on a Single Line
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Basic Syntax
Comments
A hash sign (#) that is not inside a string literal begins a comment.All characters after the # and up to the end of the physical line arepart of the comment and the Python interpreter ignores them.
1 #!/usr/bin/python323 # First comment4 print "Hello, Python!" # Second comment
Listing 3.6: Comments in Python
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Basic Syntax
Command-Line Arguments
Here’s another simple hello.py program differed from Listing 3.1:
1 #!/usr/bin/python323 # import sys modules4 import sys5 def main():6 print 'Hello there', sys.argv[1]7 # Command line args are in sys.argv[1], ...8 # sys.argv[0] is the script name itself9
10 # Standard boilerplate to call the main()11 # function to begin the program.12 if __name__ == '__main__':13 main()
Listing 3.7: Another hello.py
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Basic Syntax
Command-Line Arguments (cont.)
Running this program from the command line looks like:
$ ./hello.py SXKDZHello there SXKDZ
Listing 3.8: Result of Another hello.py
The outermost statements in a Python file, or module, do itsone-time setup – those statements run from top to bottom thefirst time the module is imported somewhere, setting up itsvariables and functions. A Python module can be run directly – asabove python hello.py SXKDZ – or it can be imported and usedby some other module.
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Basic Syntax
Command-Line Arguments (cont.)
When a Python file is run directly, the special variable __name__ isset to __main__. Therefore, it’s common to have the boilerplate if__name__ == … shown above to call a main() function when themodule is run directly, but not when the module is imported bysome other module.
In a standard Python program, the list sys.argv contains thecommand-line arguments in the standard way with sys.argv[0]being the program itself, sys.argv[1] the first argument, and soon. If you know about argc, or the number of arguments, you cansimply request this value with len(sys.argv). In general, len()can tell you how long a string is, the number of elements in listsand tuples, and the number of key-value pairs in a dictionary.
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Basic Syntax
Code Checked at Runtime
Python does very little checking at compile time, deferring almostall type, name, etc. checks on each line until that line runs.Suppose the above main() calls repeat() like this:
1 def main():2 if name == 'Guido':3 print repeeeet(name) + '!!!'4 else:5 print repeat(name)
Listing 3.9: A Python Script with Obvious Errors
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Basic Syntax
Code Checked at Runtime (cont.)
The if-statement in Listing 3.9 contains an obvious error, wherethe repeat() function is accidentally typed in as repeeeet().But this code compiles and runs fine so long as the name atruntime is not Guido. Only when a run actually tries to executethe repeeeet() will it notice that there is no such function andraise an error. This just means that when you first run a Pythonprogram, some of the first errors you see will be simple typos likethis.
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Data Types and Operators
Variables
Python variables do not need explicit declaration to reservememory space. The declaration happens automatically when youassign a value to a variable. The equal sign (=) is used to assignvalues to variables. For example:
1 counter = 100 # An integer assignment2 miles = 1000.0 # A floating point assignment3 name = "John" # A string assignment
Listing 4.1: Variable Assignments
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Data Types and Operators
Multiple Assignments
Python allows you to assign a single value or multiple values toseveral variables simultaneously. For example:
1 a, b, c = 12 d, e, f = 2, 3, "john"
Listing 4.2: Multiple Assignments
In Listing 4.2, an integer object is created with the value 1, and allthree variables a, b, c are assigned to the same memory location.Then, two integer objects with values 2 and 3 are assigned tovariables d and e respectively, and one string object with the value"john" is assigned to the variable f.
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Data Types and Operators
Built-in Types
The data stored in memory can be of many types. Python has fivebuilt-in data types:▶ number▶ string▶ list▶ tuple▶ dictionary
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Data Types and Operators
Numbers
Number data types store numeric values. Number objects arecreated when you assign a value to them.
Python supports four different numerical types:▶ int (signed integers)▶ long (long integers, they can also be represented in octal and
hexadecimal)▶ Python allows you to use a lowercase l with long, but it is
recommended that you use only an uppercase L to avoidconfusion with the number 1. Python displays long integerswith an uppercase L.
▶ float (floating point real values)▶ complex (complex numbers)
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Data Types and Operators
Strings
Python has a built-in string class named str with many handyfeatures. String literals can be enclosed by either double or singlequotes, although single quotes are more commonly used.Backslash escapes work the usual way within both single anddouble quoted literals.
The plus (+) sign is the string concatenation operator and theasterisk (*) is the repetition operator.
Python strings are immutable which means they cannot bechanged after they are created. Since strings can’t be changed, weconstruct new strings as we go to represent computed values. Sofor example the expression ('hello' + 'there') takes in the 2strings 'hello' and 'there' and builds a new string'hellothere'.
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Data Types and Operators
Strings (cont.)
Characters in a string can be accessed using the standard [ ]syntax, and Python uses zero-based indexing, so if str = 'hello'then str[1] = 'e'. If the index is out of bounds for the string,Python raises an error.
Python does not have a separate character type. Instead anexpression like s[8] returns a string of length−1 containing thecharacter. With that string, the operators ==, <=, … all work asyou would expect, so mostly you don’t need to know that Pythondoes not have a separate scalar char type.
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Data Types and Operators
Strings (cont.)
The handy slice syntax also works to extract any substring from astring. The len(string) function returns the length of a string.The [ ] syntax and the len() function actually work on anysequence type – strings, lists, etc. Python tries to make itsoperations work consistently across different types.
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Data Types and Operators
Strings (cont.)
For example:
1 #!/usr/bin/python32 str = "Hello World!"3 print(str) # Prints complete string4 print(str[0]) # Prints first character of
the string5 print(str[2:5]) # Prints characters starting
from 3rd to 5th6 print(str[2:]) # Prints string starting
from 3rd character7 print(str * 2) # Prints string two times8 print(str + "TEST")# Prints concatenated string
Listing 4.3: String Representations
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Data Types and Operators
Strings (cont.)
Listing 4.3 produces the following result:
Hello World!Hllollo World!Hello World!Hello World!Hello World!TEST
Listing 4.4: Result of String Representations
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Data Types and Operators
Quotations
Python accepts single ('), double (") and triple (''' or """)quotes to denote string literals, as long as the same type of quotestarts and ends the string. The triple quotes are used to span thestring across multiple lines. For example:
1 word = 'word'2 sentence = "This is a sentence."3 paragraph = '''This is a paragraph. It is4 made up of multiple lines and sentences.'''
Listing 4.5: String Literals in Python
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Data Types and Operators
String Methods
Here are some of the most common string methods. A method islike a function, but it runs on an object. If the variable s is astring, then the code s.lower() runs the lower() method onthat string object and returns the result.
Object-Oriented ProgrammingObject-oriented programming (OOP) is a programmingparadigm based on the concept of objects, which may containdata, in the form of fields, often known as attributes; and code, inthe form of procedures, often known as methods. A feature ofobjects is that an object’s procedures can access and often modifythe data fields of the object with which they are associated(objects have a notion of this or self).
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Data Types and Operators
String Methods (cont.)
▶ s.lower(), s.upper() – returns the lowercase or uppercaseversion of the string.
▶ s.strip() – returns a string with whitespace removed fromthe start and end.
▶ s.isalpha()/s.isdigit()/s.isspace()… – tests if all thestring chars are in the various character classes.
▶ s.startswith('other'), s.endswith('other') – tests ifthe string starts or ends with the given other string.
▶ s.find('other') – searches for the given other string (not aregular expression) within s, and returns the first index whereit begins or -1 if not found.
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Data Types and Operators
String Methods (cont.)
▶ s.replace('old', 'new') – returns a string where alloccurrences of 'old' have been replaced by 'new'.
▶ s.split('delim') – returns a list of substrings separated bythe given delimiter. The delimiter is not a regular expression,it’s just text. 'aaa,bbb,ccc'.split(',') -> ['aaa','bbb', 'ccc']. As a convenient special case s.split()(with no arguments) splits on all whitespace chars.
▶ s.join(list) – opposite of split(), joins the elements inthe given list together using the string as the delimiter, e.g.'---'.join(['aaa', 'bbb', 'ccc']) ->aaa---bbb---ccc.
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Data Types and Operators
Slices
The slice syntax is a handy way to refer to sub-parts of sequences– typically strings and lists. The slice s[start:end] is theelements beginning at start and extending up to but not includingend. Suppose we have s = "Hello":
0 1 2 3 4–5 –4 –3 –2 –1
HelloFigure 4.1: Subscripts of String Slices
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Data Types and Operators
Slices (cont.)
The standard zero-based index numbers give easy access to charsnear the start of the string. As an alternative, Python uses negativenumbers to give easy access to the chars at the end of the stringas shown in Figure 4.1: s[-1] = 'o', s[-2] = 'l', and so on.Negative index numbers count back from the end of the string.
It is a neat truism of slices that for any index n, s[:n] + s[n:]== s. This works even for n negative or out of bounds. Or putanother way s[:n] and s[n:] always partition the string into twostring parts, conserving all the characters.
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Data Types and Operators
String Formatting via the Modulo Operator
Python has a printf()-like facility to put together a string. The% operator takes a printf()-type format string on the left:▶ %d – int▶ %s – string▶ %f/%g – floating point
and the matching values in a tuple on the right:
1 # add parens to make the long-line work:2 text = ("%d little pigs come out or I'll %s and
%s and %s" %3 (3, 'huff', 'puff', 'blow down'))
Listing 4.6: String Formatting via the Modulo Operator
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Data Types and Operators
String Formatting via the format() Method
Substitute complex variable and format value in string via theformat() method (described in PEP 3101). Format stringscontain replacement fields surrounded by curly braces ({}1).With this style of formatting, it is possible to give placeholders anexplicit positional index. This allows for re-arranging the order ofdisplay without changing the arguments.
1 '{} {}'.format(1, 2) # 1 22 '{} {}'.format('one', 'two'). # one two3 '{1} {0}'.format('one', 'two') # two one4 '{:04d}'.format(42) # 00425 '{:f}'.format(3.14159265358) # 3.141593
Listing 4.7: String Formatting via the format() Method1Escaping by doubling ({{ and }}) to include a brace character in the literal
text.
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Data Types and Operators
String Formatting via Literal String Interpolation
String interpolation is a process substituting values of variablesinto placeholders in a string.
Python 3.6 added new string interpolation method called literalstring interpolation and introduced a new literal prefix f.
1 a = 12, b = 32 f'12 * 3 = {a * b}' # 12 * 3 = 363 name = 'World', program = 'Python'4 f'Hello {name}! This is {program}.' # Hello
World! This is Python.
Listing 4.8: String Formatting via Literal String Interpolation
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Data Types and Operators
Lists
Lists are the most versatile of Python’s compound data types.
A list contains items separated by commas and enclosed withinsquare brackets ([]).▶ To some extent, lists are similar to arrays in C.▶ One difference between them is that all the items belonging to
a list can be of different data types.The values stored in a list can be accessed using the slice operatoras well, with indexes starting at 0 in the beginning of the list andworking their way to end -1.
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Data Types and Operators
Lists (cont.)
For example:
1 #!/usr/bin/python32 list = ['abcd', 786 , 2.23, 'john', 70.2]3 tinylist = [123, 'john']4 print(list) # Prints complete list5 print(list[0]) # Prints first element of
the list6 print(list[1:3]) # Prints elements starting
from 2nd till 3rd7 print(list[2:]) # Prints elements starting
from 3rd element8 print(tinylist * 2) # Prints list two times9 print(list + tinylist)
10 # Prints concatenated lists
Listing 4.9: List Representation
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Data Types and Operators
Lists (cont.)
Listing 4.9 produces the following result:
['abcd', 786, 2.23, 'john', 70.2]abcd[786, 2.23][2.23, 'john', 70.2][123, 'john', 123, 'john']['abcd', 786, 2.23, 'john', 70.2, 123, 'john']
Listing 4.10: Result of List Representation
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Data Types and Operators
Lists (cont.)
Assignment with an = on lists does not make a copy. Instead,assignment makes the two variables point to the one list inmemory.
1 colors = ['red', 'blue', 'green']2 b = colors ## Does not copy the list
Listing 4.11: Referencing Lists
colorsb
'red' 'blue' 'green'
Figure 4.2: Illustration of the Memory Space
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Data Types and Operators
List Methods
Here are some common list methods:▶ list.append(elem) – adds a single element to the end of
the list. Common error: does not return the new list, justmodifies the original.
▶ list.insert(index, elem) – inserts the element at thegiven index, shifting elements to the right.
▶ list.extend(list2) adds the elements in list2 to the endof the list. Using + or += on a list is similar to usingextend().
▶ list.index(elem) – searches for the given element from thestart of the list and returns its index. Throws a ValueErrorif the element does not appear (use in to check without aValueError).
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Data Types and Operators
List Methods (cont.)
▶ list.remove(elem) – searches for the first instance of thegiven element and removes it (throws ValueError if notpresent).
▶ list.sort() – sorts the list in place.▶ list.reverse() – reverses the list in place.▶ list.pop(index) – removes and returns the element at the
given index. Returns the rightmost element if index is omitted(roughly the opposite of append()).
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Data Types and Operators
Tuples
A tuple is a fixed size grouping of elements, such as an (x, y)co-ordinate. Tuples are like lists, except they are immutable and donot change size (tuples are not strictly immutable since one of thecontained elements could be mutable). Unlike lists, tuples areenclosed within parentheses.
Tuples play a sort of “struct” role in Python – a convenient way topass around a little logical, fixed size bundle of values. Forexample, if I wanted to have a list of 3-d coordinates, the naturalpython representation would be a list of tuples, where each tuple issize 3 holding one (x, y, z) group.
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Data Types and Operators
Tuples (cont.)
To create a size-1 tuple, the lone element must be followed by acomma:
1 tuple = ('hi',) ## size-1 tuple
Listing 4.12: A Size-1 Tuple
The comma is necessary to distinguish the tuple from the ordinarycase of putting an expression in parentheses. In some cases youcan omit the parenthesis and Python will see from the commasthat you intend a tuple.
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Data Types and Operators
Tuples (cont.)
The following code is invalid with tuple, because it is not allowedto update a tuple. Similar case is possible with lists:
1 #!/usr/bin/python323 tuple = ('abcd', 786, 2.23, 'john', 70.2)4 list = ['abcd', 786, 2.23, 'john', 70.2]5 tuple[2] = 1000 # Invalid syntax with tuple6 list[2] = 1000 # Valid syntax with list
Listing 4.13: Invalid Tuple Operations
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Data Types and Operators
Dictionaries
Python’s efficient key/value hash table structure is called a dict.The contents of a dict can be written as a series of key:valuepairs within braces { }, e.g., dict = {key1:value1,key2:value2, …}.
Looking up or setting a value in a dict uses square brackets, e.g.,dict['foo'] looks up the value under the key 'foo'. Strings,numbers, and tuples work as keys, and any type can be a value.Other types may or may not work correctly as keys (strings andtuples work cleanly since they are immutable). Looking up a valuewhich is not in the dict throws a KeyError – use in to check ifthe key is in the dict, or use dict.get(key) which returns thevalue or None if the key is not present (or get(key, not-found)allows you to specify what value to return in the not-found case).
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Data Types and Operators
Dictionaries (cont.)
For example:
1 dict = {}2 dict['a'] = 'alpha'3 dict['g'] = 'gamma'4 print(dict)5 ## {'a': 'alpha', 'o': 'omega', 'g': 'gamma'}6 print(dict['a']) ## Simple lookup: 'alpha'7 dict['a'] = 6 ## Put new key/value into
dict8 'a' in dict ## True9 ## print(dict['z']) ## Throws KeyError
10 if 'z' in dict:11 print(dict['z']) ## Avoid KeyError12 print(dict.get('z')) ## None (avoid KeyError)
Listing 4.14: An Example Using Dict
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Data Types and Operators
Dictionaries (cont.)
'foo1'
'foo2'
'foo3'
keys
dict
'bar1'
'bar2'
'bar3'
values
Figure 4.3: Illustration of the Dict Memory Space
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Data Types and Operators
Dictionaries (cont.)
The methods dict.keys() and dict.values() return lists ofthe keys or values explicitly. There’s also an items() whichreturns a list of (key, value) tuples, which is the most efficientway to examine all the key value data in the dictionary.
Strategy NoteFrom a performance point of view, the dictionary is one of yourgreatest tools, and you should use it where you can as an easy wayto organize data. For example, you might read a log file where eachline begins with an IP address, and store the data into a dict usingthe IP address as the key, and the list of lines where it appears asthe value. Once you’ve read in the whole file, you can look up anyIP address and instantly see its list of lines. The dictionary takes inscattered data and makes it into something coherent.
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Data Types and Operators
Dictionary Formatting
Both formatting styles support named placeholders to convenientlyto substitute values from a dict into a string by name:
1 d = {}2 d['word'] = 'garfield'3 d['cnt'] = 424 s = 'I want %(cnt)d copies of %(word)s' % d #
old style5 s = 'I want {cnt} copies of {word}'.format(**d)
# new style6 # 'I want 42 copies of garfield'
Listing 4.15: Dictionary Value Substitution
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Data Types and Operators
Data Type Conversion
Sometimes, you may need to perform conversions between thebuilt-in types. To convert between types, you simply use the typename as a function.
There are several built-in functions to perform conversion from onedata type to another. These functions return a new objectrepresenting the converted value.
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Data Types and Operators
Data Type Conversion (cont.)
Function Descriptionint(x [,base]) Converts x to an integer. base specifies the
base if x is a string.long(x [,base]) Converts x to a long integer. base specifies
the base if x is a string.float(x) Converts x to a floating-point number.str(x) Converts object x to a string representation.tuple(s) Converts s to a tuple.list(s) Converts s to a list.dict(d) Creates a dictionary. d must be a sequence
of (key,value) tuples.
Table 4.1: Some Useful Type Conversion Functions
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Data Types and Operators
Basic Operators
Operators are the constructs which can manipulate the value ofoperands.
Python language supports the following types of operators:▶ Arithmetic Operators▶ Comparison (Relational) Operators▶ Assignment Operators▶ Logical Operators▶ Bitwise Operators▶ Membership Operators▶ Identity Operators
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Data Types and Operators
Arithmetic Operators
Basic arithmetic operators are the following:▶ + (Addition) – Adds values on either side of the operator.▶ - (Subtraction) – Subtracts right hand operand from left
hand operand.▶ * (Multiplication) – Multiplies values on either side of the
operator.▶ / (Division) – Divides left hand operand by right hand
operand.▶ % (Modulus) – Divides left hand operand by right hand
operand and returns remainder.▶ ** (Exponent) – Performs exponential (power) calculation on
operators.
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Data Types and Operators
Arithmetic Operators (cont.)
▶ // (Floor Division) – The division of operands where theresult is the quotient in which the digits after the decimalpoint are removed. But if one of the operands is negative, theresult is floored, i.e., rounded away from zero (towardsnegative infinity):▶ 9//2 = 4▶ 9.0//2.0 = 4.0▶ −11//3 = −4▶ −11.0//3 = −4.0
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Data Types and Operators
Comparison Operators
These operators compare the values on either sides of them anddecide the relation among them. They are also called Relationaloperators.▶ == – If the values of two operands are equal, then the
condition becomes true.▶ != – If values of two operands are not equal, then condition
becomes true.▶ <> – If values of two operands are not equal, then condition
becomes true. This is similar to != operator.
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Data Types and Operators
Comparison Operators (cont.)
▶ > – If the value of left operand is greater than the value ofright operand, then condition becomes true.
▶ < – If the value of left operand is less than the value of rightoperand, then condition becomes true.
▶ >= – If the value of left operand is greater than or equal tothe value of right operand, then condition becomes true.
▶ <= – If the value of left operand is less than or equal to thevalue of right operand, then condition becomes true.
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Data Types and Operators
Assignment Operators
Assignment operators are quite like C. Python supports = as thebasic assignment operator and also adding-assignment operator +=.Apparently, there’re 7 corresponding assignment operators alongwith their arithmetic operators.
▶ Value stored in computer memory.▶ An assignment binds name to value.▶ Retrieve value associated with name or variable by invoking
the name.
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Data Types and Operators
Bitwise Operators
Bitwise operator works on bits and performs bit by bit operation.Assume if a = 60 and b = 13. Now in binary format they will be asfollows:
a = (0011 1100)2b = (0000 1101)2a&b = 0000 1100a|b = 0011 1101aˆb = 0011 0001˜a = 1100 0011
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Data Types and Operators
Identity Operators
Identity operators compare the memory locations of two objects.There are two Identity operators explained below:
Operator Description Example
is
Evaluates to True if thevariables on either side of theoperator point to the sameobject and False otherwise.
x is y results inTrue if id(x)equals to id(y).
is not
Evaluates to False if thevariables on either side of theoperator point to the sameobject and True otherwise.
x is not yresults in True ifid(x) doesn’tequal to id(y).
Table 4.2: Identity Operators
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Data Types and Operators
The Deleting Operator
The del operator does deletions. In the simplest case, it canremove the definition of a variable, as if that variable had not beendefined. It can also be used on list elements or slices to delete thatpart of the list and to delete entries from a dictionary.
1 var = 62 del var # var no more3 list = ['a', 'b', 'c', 'd']4 del list[0] ## Delete first element5 del list[-2:] ## Delete last two elements6 print(list) ## ['b']7 dict = {'a':1, 'b':2, 'c':3}8 del dict['b'] ## Delete 'b' entry9 print(dict) ## {'a':1, 'c':3}
Listing 4.16: An Example Using del
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Control Statements
if Statement
Python does not use {} to enclose blocks of code for branches,loops, functions, etc. Instead, Python uses the colon (:) andindentation/whitespace to group statements. The boolean test foran if does not need to be in parenthesis, and it can have elifand else clauses.MnemonicThe word elif is the same length as the word else.
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Control Statements
if Statement (cont.)
Any value can be used as an if-test. The “zero” values all countas false: None, 0, empty string, empty list, empty dictionary.There is also a Boolean type with two values: True and False(converted to an int, these are 1 and 0).
Python has the usual comparison operations: ==, !=, <, <=, >,>=. Unlike Java and C, == is overloaded to work correctly withstrings. The boolean operators are the spelled out words and, or,not (Python does not use the C-style &&, ||, !).
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Control Statements
if Statement (cont.)
Here’s what the code might look like for a policeman pulling over aspeeder:
1 if speed >= 80:2 print('License and registration please.')3 if mood == 'terrible' or speed >= 100:4 print('You have the right to remain silent.')5 elif mood == 'bad' or speed >= 90:6 print("I'm going to have to write you a
ticket.")7 write_ticket()8 else:9 print("Let's try to keep it under 80 ok?")
Listing 5.1: An Example Using if Statement
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Control Statements
if Statement (cont.)
If the code is short, you can put the code on the same line after :,like this (this also applies to functions, loops, etc.):
1 if speed >= 80: print('You are so busted')2 else: print('Have a nice day')
Listing 5.2: A Shorter Example Using if Statement
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Control Statements
in Statement
The in construct on its own is an easy way to test if an elementappears in a list (or other collection) – value in collection –tests if the value is in the collection, returning True/False.
1 list = ['larry', 'curly', 'moe']2 if 'curly' in list:3 print('yay')
Listing 5.3: An Example Using in Statement
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Control Statements
Ranged Loops
Python’s for and in constructs are extremely useful. The forconstruct – for var in list – is an easy way to look at eachelement in a list (or other collection). Do not add or remove fromthe list during iteration. For example:
1 squares = [1, 4, 9, 16]2 sum = 03 for num in squares:4 sum += num5 print(sum) ## 30
Listing 5.4: Ranged Loops
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Control Statements
Range Generating
The range(n) function yields the numbers 0, 1, …, n−1, andrange(a, b) returns a, a+1, …, b−1 – up to but not includingthe last number. The combination of the for-loop and therange() function allow you to build a traditional numeric for loop:
1 # Generates the Numbers from 0 through 992 for i in range(100):3 print(i)
Listing 5.5: An Example of range() Function
There is a variant xrange() which avoids the cost of building thewhole list for performance sensitive cases.
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Control Statements
while Loop
Python also has the standard while-loop, and the break andcontinue statements work as in C++ and Java, altering thecourse of the innermost loop. The above for/in loops solves thecommon case of iterating over every element in a list, but thewhile loop gives you total control over the index numbers. Here’s awhile loop which accesses every 3rd element in a list:
1 ## Access every 3rd element in a list2 i = 03 while i < len(a):4 print(a[i])5 i = i + 3
Listing 5.6: An Example of while Loop
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Functions
Defining a Function
A function is a block of organized, reusable code that is used toperform a single, related action. Functions provide bettermodularity for your application and a high degree of code reusing.
The def keyword defines the function with its parameters withinparentheses and its code indented. The first line of a function canbe a documentation string (docstring) that describes what thefunction does. Variables defined in the function are local to thatfunction. The return statement can take an argument, in whichcase that is the value returned to the caller.
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Functions
Defining a Function (cont.)
1 # Defines a "repeat" function that takes 2arguments.
2 def repeat(s, exclaim):3 """4 Returns the string 's' repeated 3 times.5 If exclaim is true, add exclamation marks.6 """78 result = s * 39 if exclaim:
10 result = result + '!!!'11 return result
Listing 6.1: Function Definitions
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Modules
More on Modules and their Namespaces
Suppose you’ve got a module binky.py which contains a deffoo(). The fully qualified name of that foo function isbinky.foo. In this way, various Python modules can name theirfunctions and variables whatever they want, and the variable nameswon’t conflict – module1.foo is different from module2.foo.
In the Python vocabulary, we’d say that binky, module, andmodule2 each have their own namespaces, which as you canguess are variable name-to-object bindings.
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Modules
More on Modules and their Namespaces (cont.)
For example, we have the standard sys module that contains somestandard system facilities, like the argv list, and exit() function.With the statement import sys you can then access thedefinitions in the sys module and make them available by theirfully-qualified name, e.g. sys.exit().
1 import sys23 # Now can refer to sys.xxx facilities4 sys.exit(0)
Listing 7.1: Importing sys Module
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Modules
More on Modules and their Namespaces (cont.)
There is another import form that looks like this: from sysimport argv, exit. That makes argv and exit() available bytheir short names; however, we recommend the original form withthe fully-qualified names because it’s a lot easier to determinewhere a function or attribute came from.
There are many modules and packages which are bundled with astandard installation of the Python interpreter, so you don’t haveto do anything extra to use them. These are collectively known asthe Python Standard Library.
You can find the documentation of all the Standard Librarymodules and packages at https://docs.python.org/library.
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Exceptions
Exceptions
An exception represents a runtime error that halts the normalexecution at a particular line and transfers control to error handlingcode. This section just introduces the most basic uses ofexceptions. For example a runtime error might be that a variableused in the program does not have a value (ValueError), or a fileopen operation error because that a does not exist (IOError) Seeexception docs for more useful information.
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Exceptions
Exceptions (cont.)
Without any error handling code, a runtime exception just haltsthe program with an error message. That’s a good defaultbehavior, and you’ve seen it many times. You can add atry/except structure to your code to handle exceptions, like this:
1 try:2 f = open(filename , 'rU')3 text = f.read()4 f.close()5 except IOError:6 ## Control jumps directly to here if any of
the above lines throws IOError.7 sys.stderr.write('problem reading:' +
filename)
Listing 8.1: An Example Handling Exceptions
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Exceptions
Exceptions (cont.)
▶ The try: section includes the code which might throw anexception.
▶ The except: section holds the code to run if there is anexception.
▶ If there is no exception, the except: section is skipped (thatis, that code is for error handling only, not the “normal” casefor the code). You can get a pointer to the exception objectitself with syntax except IOError, e: … (e points to theexception object).
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Closing Up and Further More
Getting Help
There are a variety of ways to get help for Python.▶ The official Python docs site has high quality docs.▶ The official Tutor mailing list specifically designed for those
who are new to Python and/or programming.▶ Many questions (and answers) can be found on StackOverflow
and Quora.▶ Use the help() and dir() functions.
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Closing Up and Further More
The help() and dir() Function
Inside the Python interpreter, the help() function pulls updocumentation strings for various modules, functions, andmethods. These doc strings are similar to Java’s javadoc. Thedir() function tells you what the attributes of an object are.Below are some ways to call help() and dir() from theinterpreter:▶ help(len) – help string for the built-in len() function; note
that it’s len not len(), which is a call to the function.▶ dir(sys) – dir() is like help() but just gives a quick list of
its defined symbols, or attributes.▶ help(sys.exit) – help string for the exit() function in the
sys module.
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Closing Up and Further More
The help() and dir() Function (cont.)
▶ help('xyz'.split) – help string for the split() methodfor string objects. You can call help() with that object itselfor an example of that object, plus its attribute. For example,calling help('xyz'.split) is the same as callinghelp(str.split).
▶ help(list) – help string for list objects.▶ dir(list) – displays list object attributes, including its
methods.▶ help(list.append) – help string for the append() method
for list objects.
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References
References
▶ ’s Python Class by Nick Parlante▶ Python Official Beginners’ Guide▶ Massachusetts Institute of Technology 6.0001: Introduction to
Computer Science and Programming in Python
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Copyright Information
Copyright Information
Except as otherwise noted, the content is under the CreativeCommons Attribution-ShareAlike License 3.0 cba, and codesamples are licensed under the Apache 2.0 License.
Java is a registered trademark of Oracle and/or its affiliates.
