Why Python 3

Why Python 3!Finally Possible

Wednesday, October 10, 12

Why We Didn’t Switch

Python 3 is incompatible with Python 2

Performance was worse

Memory usage was higher

Most libraries and frameworks were not ported


Performance

Python 3.0 had a whole new IO library

Much cleaner, but...

Entirely written in Python

Fixed in Python 3.1


Memory Usage

Mainly caused by all-Unicode all-the-time

Every string took 2 or 4 bytes per character

Often memory requirements for an app doubled or worse

Fixed in Python 3.3


Libraries and Frameworks

NumPy 1.5+ and SciPy 0.9+

Django 1.5+

Pyramid 1.3+

Six library used as a compatibility layer by many libraries now


OK, But Why Bother?

Cleaned up syntax, better library organization, blah, blah, blah. But it does make things better going forward.

Simpler string handling

Lot’s of cool new features


Cleaned Up Syntax

except Exception,e -> except Exception as e

True, False and None are reserved words now. Whew, that could have been ugly!

All classes are “new” classes

super() instead super(parent,self)

Annotations are cool!


Simpler String Handling

All strings are Unicode

No more worrying about codecs

Flexible String Representation in Python 3.3 solves memory issues


More Comprehensions

There are now dict and set comprehensions

{k: v for k, v in stuff if v} (Python 2.7+)

{k for k in list_of_stuff if k>0}


New Syntax Tricks

Sets: {1,2,3}

Keyword only arguments:def awesome(name, *, pumpItUp=False)

Extended unpacking:a, b, *rest = range(5)

nonlocal is like global but for the enclosure


Chained Exceptions

try: a = divide(1,0)except Exception as e: raise ThatsNotGood() from e

Traceback shows both exceptions

Both exceptions also shown if you have an exception in your exception handler


Ordered Dictionaries

Normal dictionaries are random order

Can be a real problem for reading and writing JSON since keys get scrambled

Ordered Dictionaries preserve insertion order

Used by the JSON library


Formatting Numbers

Easy to get commas now:

format(1234567, ‘,d’) -> ‘1,234,567’

format(1234567.89, ‘,.2f’) -> ‘1,234,567.89’


Context Manager

Back-ported to Python 2.6 so use it now!

Manages lifetime. Use instead of try/finally.

with open(‘mylog.txt’) as infile: error_lines=[] for line in infile: if ‘ERROR’ in line: error_lines.append(line)

File closed as soon as with block ends


contextlibfrom contextlib import contextmanager

@contextmanagerdef db_transaction(connection): cursor = connection.cursor() try: yield cursor except: connection.rollback() raise else: connection.commit()

db = DatabaseConnection()with db_transaction(db) as cursor: ...


argparse

Much better library for handling command-line arguments

Automatically generates help

Available for Python 2.6 so use it now

Really, check it out!


concurrent.futures

Simplifies multithreading:

import concurrent.futures, shutil

with concurrent.futures.ThreadPoolExecutor(max_workers=4) as e: e.submit(shutil.copy, 'src1.txt', 'dest1.txt') e.submit(shutil.copy, 'src2.txt', 'dest2.txt') e.submit(shutil.copy, 'src3.txt', 'dest3.txt') e.submit(shutil.copy, 'src4.txt', 'dest4.txt')


New GIL

GIL is the great Achilles Heal of Python

Old GIL had pretty bad performance on multicore boxes

New GIL in Python 3.2 is generally much better. Performance was enough better for me to abandon gevent!

Can still be ugly if you mix CPU-bound and IO-bound code


Annotations

Let you add some purely documentary tags onto parameters and functions:

def digest(stuff: “iterable of numbers”, non_zero:bool=False) -> hash:


New in Python 3.3

venv - virtualenv is baked in now

Fancy new I/O exceptions. Instead of OSError, now we get things like:

PermissionError

FileNotFoundError

More compact attribute dictionaries


Biggest Gotcha Award

‘print’ is a function now

dict.keys(), dict.values() and dict.items() return generators instead of lists


Questions?

Want to see an argparse example?

Email me: [email protected]


mailto:[email protected]

mailto:[email protected]

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