Python 2 to 3 Quick Guide

Few years ago I was programming Python 2.7, when 3.x was still not an option, because of its backward-incompatibiliy and lack of popular third-party libraries support. But now it’s safe to say Python 3 is totally ready, and here’s a list of references for those (including me) who are adopting Python 3 with a 2.x background.

  1. All Strings Are Unicode
  2. print Becomes a Function
  3. Less Lists More Views
  4. Integer Division Returns Float
  5. Comparison Operators Raises TypeError
  6. Set Literal Support
  7. New String Formatting
  8. Exception Handling
  9. Global Function Changes
  10. Renaming Modules and Relative Import

All Strings Are Unicode

When dealing with non-ASCII encodings in Python 2, there’re str, unicode, u'...', s.encode(), etc. In Python 3, there’re only text and binary data. The former is str, strings that are always represented in Unicode; the later is bytes, which is just a sequence of byte numbers.

  • Conversion between str and bytes:
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# str to bytes
'str'.encode('UTF-8')
bytes('str', encoding='UTF-8')
# bytes to str
b'bytes'.decode('UTF-8')
str(b'bytes', encoding='UTF-8')
  • basestring is removed, use str as type: isinstance(s, str)
  • bytes is immutable, the corresponding mutable version is bytearray.
  • The default source file encoding is UTF-8 now.

In Python 2, print is a statement, and now it’s used as a function:

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print # Old: print a new line
print() # New
print 'hello', 'world', # Old: trailing comma suppresses new line
print('hello', 'world', end=' ') # New: end defaults to '\n'
print >>sys.stderr, 'error' # Old: write to stderr
print('error', file=sys.stderr) # New

print function also provides sep and flush parameters:

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print('hello', 'world', sep=',', flush=True)
# Instead of:
print ','.join(('hello', 'world'))
sys.stdout.flush()

Less Lists More Views

A lot of well-known methods now return iterators, or ‘views’, instead of eager-evaluated lists.

  • Dictionary’s keys, items, and values methods, while removing iterkeys, iteritems, and itervalues. For example, when you need a sorted key list:
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k = d.keys(); k.sort() # Old
k = sorted(d.keys()) # New
  • map, filter, and zip, while removing imap methods in itertools module. To get a concrete list, use list comprehension or the list global function:
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[x * 2 for x in [1, 2, 3]]
list(map(lambda x: x * 2, [1, 2, 3]))
  • range is now equivalent to xrange in Python 2, the later is removed.
  • For iterators, the next method is renamed to __next__, and there’s a global next function, which accepts an iterator and calls its __next__ method.
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iter([1]).next() # Old
iter([1]).__next__() # New
next(iter([1])) # New

Integer Division Returns Float

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print 1 / 2 # Old: prints 0
print 1 / 2.0 # Old: prints 0.5
print(1 / 2) # New: prints 0.5
print(1 // 2) # New: prints 0
  • There’s no difference between long and int now, use int only.
  • Octal literals are represented as 0o755, instead of 0755.

Comparison Operators Raises TypeError

  • <, <=, >=, > can no longer be used between different types.
  • == and != remains the same.
  • cmp parameter in sort is removed. Use key to extract a comparison key from each element.

Set Literal Support

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s = set([1, 2, 3]) # Old, also valid in Python 3
s = {1, 2, 3} # New
s = set() # Empty set
d = {} # Empty dict

New String Formatting

Python 3 introduces a new form of string formatting, and it’s also back-ported to Python 2.x. The old %s formatting is still available in 3.x, but the new format seems more expressive and powerful.

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# by position
'{}, {}, {}'.format('a', 'b', 'c') # a, b, c
'{2}, {1}, {0}'.format('a', 'b', 'c') # c, b, a
# by name
'Hello, {name}'.format(name='Jerry') # Hello, Jerry
# by attribute
c = 1 - 2j
'real: {0.real}'.format(c) # real: 1.0
# by index
'X: {0[0]}, Y: {0[1]}'.format((1, 2)) # X: 1, Y: 2
# format number
'{:.2f}'.format(1.2) # 1.20
'{:.2%}'.format(0.012) # 1.20%
'{:,}'.format(1234567) # 1,234,567
# padding
'{:>05}'.format(1) # 00001

Furthermore, Python 3.6 introduces literal string interpolation (PEP 498).

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name = 'Jerry'
print(f'Hello, {name}')

Exception Handling

Raise and catch exceptions in a more standard way:

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# Old
try:
raise Exception, 'message'
except Exception, e:
tb = sys.exc_info()[2]
# New
try:
raise Exception('message')
except Exception as e:
tb = e.__traceback__

Global Function Changes

Some global functions are (re)moved to reduce duplication and language cruft.

  • reduce is removed, use functools.reduce, or explict for loop instead.
  • apply is removed, use f(*args) instead of apply(f, args).
  • execfile is removed, use exec(open(fn).read())
  • Removed backticks, use repr instread.
  • raw_input is renamed to input, and the old input behaviour can be achieved by eval(input())

Renaming Modules and Relative Import

  • Different URL modules are unified into urllib module, e.g.
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from urllib.request import urlopen, Request
from urllib.parse import urlencode
req = Request('http://shzhangji.com?' + urlencode({'t': 1})
with urlopen(req) as f:
print(f.read())
  • Some modules are renamed according to PEP 8, such as:
    • ConfigParser -> configparser
    • copy_reg -> copyreg
    • test.test_support -> test.support
  • Some modules have both pure Python implementation along with an accelerated version, like StringIO and cStringIO. In Python 3, user should always import the standard module, and fallback would happen automatically.
    • StringIO + cStringIO -> io
    • pickle + cPickle -> pickle
  • All import forms are interpreted as absolute imports, unless started with .:
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from . import somemod
from .somemod import moremod

References