Python 2-to-3 Cheat Sheet

A short reference of new Python 3 features.

Python 3.4

• PEP 446: Newly created file descriptors non-inheritable by default.
• min(..., default=...) and max(..., default=...).
• asyncio. Status: Provisional.
• enum.
• pathlib. Status: Provisional.

Python 3.3

• yield from statement:
  • Receive sent and thrown values from the calling scope.
  • Return value to the outer generator.
• venv module and pyvenv script.
• raise exc from None to suppress exception context.
• collections.ChainMap.

Python 3.2

• optparse argparse.
• str.format_map().
• @functools.lru_cache(maxsize=...).
• @functools.total_ordering.
• collection.OrderedDict.move_to_end(key, last=True).
• threading.Barrier.
• io.BytesIO.getbuffer() returns an editable view of the internal data.
• @reprlib.recursive_repr() for recursive structure.
• with subprocess.Popen(...).
• with tempfile.TemporaryDirectory().

Python 3.1

• collections.OrderedDict, which is also used by configparser, collections.namedtuple, json, etc.
• collections.Counter.
• format(1234567.89, ',.2f') == '1,234,567.89'.
• n.bit_length().
• Multiple context managers:

with open('input') as infile, open('output', 'w') as outfile:

• collections.namedtuple(..., rename=False).
• logging.NullHandler.
• importlib.

Python 3.0

• print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False).
• exec() is a function.
• dict.keys(), dict.items(), and dict.values() return views.
• dict.iterkeys() dict.iteritems() dict.itervalues()
• map() and filter() return iterators.
• map() stops when the shortest iterator is exhausted.
• xrange() range().
• zip() returns an iterator.
• You cannot make comparison of None < None, 1 < '', etc.
• list.sort() and sorted(): cmp key.
• object.__cmp__().
• long int.
• 1 / 2 == 0.5 and 4 // 2 == 1.
• sys.maxint sys.maxsize (since “maxint” makes not sense anymore).
• Octal literal: 0o720. No 0720 anymore. oct(16) == '0o20'.
• Binary literal: 0b101. bin(8) == '0b1000'.
• str is a sequence of Unicode code points.
• str(byte_string, encoding=...) vs bytes(text_string, encoding=...).
• 'Unicode string literal' vs b'byte string literal'.
• basestring (since there is not unicode and str now).
• Remember to pass encoding to open(..., encoding=None, ...) when opening in text mode.
• sys.stdin et al. are now Unicode-only text files (io.TextIOBase). Use io.TextIOBase.bufer for byte data.
• File names and paths are now Unicode strings, but:
  • Some APIs accept byte strings, too.
  • Some APIs return byte strings when passing in byte strings, too.
• os.environ, sys.argv, etc. might not be interpretable using the current encoding.
• StringIO cStringIO io.StringIO and io.BytesIO.
• PEP 3107: Argument and return value annotations.

# func.__annotations__ == {'return': 'world', 'arg': 'hello'}
def func(arg: 'hello') -> 'world':
    pass

• PEP 3102: Keyword-only arguments.

# Not required to have a default value.
def func(*args, keyword_only):
  pass

def func(*, keyword_only):
  pass

• PEP 3104: nonlocal, lexical scoping.
• PEP 3132: a, b, *rest = any_iterable or *rest, c = stuff.
• PEP 0274: Dictionary comprehensions {k : v for k, v in stuff} and set comprehensions {e for e in stuff}.
• Set literal {1, 2}. {} is an empty dictionary.
• PEP 3109 and PEP 3134: Exception chaining:

# Re-raise the active exception.
raise
# Implicit exception chaining.
# e.__context__
raise E(V)
# Explicit exception chaining.
# e.__cause__ == C
raise E(V) from C

• True, False, and None are reserved words.
• except exc as var.
• object.__metaclass__ class C(metaclass=M).
• Ellipsis ... is a built-in constant.
• Classic classes are gone.
• All import forms not starting with . are interpreted as absolute imports.
• PEP 3129: Class decorators.