# Conversion

2to3

-w // modify file; backup is created: file.py.bak

-n // do not create backup file.py.bak

-o dir // write modified file to dir

3to2

http://python-future.org // write python3 with back compatibility

# print is function, no statement

print x // does no longer work

print(x, end=' ', file=sys.stderr)

# Extended unpacking

a, b = range(2)

a, b, *rest = range(10)

*rest, a, b = range(10)

first, *t, last = open(file).read().splitlines()

def keyword_only(*many_values, sum=True)

def keyword_only(two, values, *, sum=True)

## interpretation

*list // unpack values of list

l = [1, 2]

print(*l) // == print(l[0], l[1])

a, *l, b = values // == a, l[0], l[1], b = values

string.format(*l) // ==string.format(l[0], l[1])

**dict // unpack values of dict

d = {'a':1, 'b':2}

'{b} {a}'.format(**d) // == '{b} {a}'.format(a=d['a'], b=d['b'])

# Dictionary comprehensives

t = {('a', 1), ('b', 2)}

d = dict(t)

d = {k:v for k,v in t}

# Function that return iterator instead of list

range()

map(), filter(), zip()

d.keys(), d.values()

list(iterator) // iterator -> list

# Creating own iterator

def get_numbers(n):

for i in range(n):

yield i * 2

def get_numbers(n):

yield from range(n)

# Integers

long -> int // int has variable length; long does no longer exist

1 / 2 = 0.5 // integer division returns float

1 // 2 = 0 // floor division

# Enum

from enum import Enum

class Colors(Enum):

red = 1

blue = 1 // values may not be unique

green = 3

Colors.red

Colors.red.name

Colors.red.value

Colors['green'] -> green

Colors(3) -> green

for color in Colors:

print(color.name)

# Unicode strings

* unicode: code points

'h' -> 68

'r' -> 72

'hr' -> 6872

* code points can be stored in different byte formats

* bytes: bytes as stored on disk

* bytes / str completely separated in python3

* bytes / str can be mixed in python2

'Hello' = str('Hello') => unicode in python3

'Hello' = str('Hello') => bytes in python2

bytes() for binary data

s = str('Hello world')

b = b'binary data'

b = s.encode() // unicode -> bytes

s = b.decode() // bytes -> unicode

'123' + b'123' // exception; mixing unicode and bytes not allowed in python3

'123' + b'123'.decode() // works

open(file) // returns unicode/str in python3; bytes in python2

open(file, 'b') // returns bytes

# dict.iter() / iteritems()

## Python2

.keys(), .values(), .items() // returns list

.iterkeys(), .itervalues(), .iteritems() // returns iterator

## Python3

.iterX() does not exist anymore

.keys() // return dict_keys: list OR iterator

for k in d.keys()

* returns iterator implicitly

* == for k in iter(d.keys())

k = d.keys() // return dict_keys(), like list (?)

## six

import six

from six.moves import range

for k, v in six.iteritems(d) // iterator

for v in six.itervalues(d) // iterator

for k in six.iterkeys(d) // iterator

for i in range(10):

six.PY2 / six.PY3 // check Python version

## futures

from __future__ import division

from __future__ import absolute_import

from __future__ import print_function

from __future__ import unicode_literals