2to3 is a Python program that reads Python 2.x source code and applies a series
of fixers to transform it into valid Python 3.x code. The standard library
contains a rich set of fixers that will handle almost all code. 2to3 supporting
library lib2to3
is, however, a flexible and generic library, so it is
possible to write your own fixers for 2to3. lib2to3
could also be
adapted to custom applications in which Python code needs to be edited
automatically.
2to3 will usually be installed with the Python interpreter as a script. It is
also located in the Tools/scripts
directory of the Python root.
2to3’s basic arguments are a list of files or directories to transform. The directories are recursively traversed for Python sources.
Here is a sample Python 2.x source file, example.py
:
def greet(name):
print "Hello, {0}!".format(name)
print "What's your name?"
name = raw_input()
greet(name)
It can be converted to Python 3.x code via 2to3 on the command line:
$ 2to3 example.py
A diff against the original source file is printed. 2to3 can also write the
needed modifications right back to the source file. (A backup of the original
file is made unless -n
is also given.) Writing the changes back is
enabled with the -w
flag:
$ 2to3 -w example.py
After transformation, example.py
looks like this:
def greet(name):
print("Hello, {0}!".format(name))
print("What's your name?")
name = input()
greet(name)
Comments and exact indentation are preserved throughout the translation process.
By default, 2to3 runs a set of predefined fixers. The
-l
flag lists all available fixers. An explicit set of fixers to run
can be given with -f
. Likewise the -x
explicitly disables a
fixer. The following example runs only the imports
and has_key
fixers:
$ 2to3 -f imports -f has_key example.py
This command runs every fixer except the apply
fixer:
$ 2to3 -x apply example.py
Some fixers are explicit, meaning they aren’t run by default and must be
listed on the command line to be run. Here, in addition to the default fixers,
the idioms
fixer is run:
$ 2to3 -f all -f idioms example.py
Notice how passing all
enables all default fixers.
Sometimes 2to3 will find a place in your source code that needs to be changed, but 2to3 cannot fix automatically. In this case, 2to3 will print a warning beneath the diff for a file. You should address the warning in order to have compliant 3.x code.
2to3 can also refactor doctests. To enable this mode, use the -d
flag. Note that only doctests will be refactored. This also doesn’t require
the module to be valid Python. For example, doctest like examples in a reST
document could also be refactored with this option.
The -v
option enables output of more information on the translation
process.
Since some print statements can be parsed as function calls or statements, 2to3
cannot always read files containing the print function. When 2to3 detects the
presence of the from __future__ import print_function
compiler directive, it
modifies its internal grammar to interpret print()
as a function. This
change can also be enabled manually with the -p
flag. Use
-p
to run fixers on code that already has had its print statements
converted.
The -o
or --output-dir
option allows specification of an
alternate directory for processed output files to be written to. The
-n
flag is required when using this as backup files do not make sense
when not overwriting the input files.
New in version 3.2.3: The -o
option was added.
The -W
or --write-unchanged-files
flag tells 2to3 to always
write output files even if no changes were required to the file. This is most
useful with -o
so that an entire Python source tree is copied with
translation from one directory to another.
This option implies the -w
flag as it would not make sense otherwise.
New in version 3.2.3: The -W
flag was added.
The --add-suffix
option specifies a string to append to all output
filenames. The -n
flag is required when specifying this as backups
are not necessary when writing to different filenames. Example:
$ 2to3 -n -W --add-suffix=3 example.py
Will cause a converted file named example.py3
to be written.
New in version 3.2.3: The --add-suffix
option was added.
To translate an entire project from one directory tree to another use:
$ 2to3 --output-dir=python3-version/mycode -W -n python2-version/mycode
Each step of transforming code is encapsulated in a fixer. The command 2to3 -l
lists them. As documented above, each can be turned on
and off individually. They are described here in more detail.
apply
apply()
. For example apply(function, *args, **kwargs)
is converted to function(*args, **kwargs)
.asserts
Replaces deprecated unittest
method names with the correct ones.
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basestring
basestring
to str
.buffer
buffer
to memoryview
. This fixer is optional because the memoryview
API is similar but not exactly the same as that of buffer
.dict
dict.iteritems()
is converted to dict.items()
, dict.iterkeys()
to dict.keys()
, and dict.itervalues()
to dict.values()
. Similarly, dict.viewitems()
, dict.viewkeys()
and dict.viewvalues()
are converted respectively to dict.items()
, dict.keys()
and dict.values()
. It also wraps existing usages of dict.items()
, dict.keys()
, and dict.values()
in a call to list
.except
except X, T
to except X as T
.exec
exec
statement to the exec()
function.execfile
execfile()
. The argument to execfile()
is wrapped in calls to open()
, compile()
, and exec()
.exitfunc
sys.exitfunc
to use of the atexit
module.funcattrs
my_function.func_closure
is converted to my_function.__closure__
.future
from __future__ import new_feature
statements.getcwdu
os.getcwdu()
to os.getcwd()
.has_key
dict.has_key(key)
to key in dict
.idioms
This optional fixer performs several transformations that make Python code
more idiomatic. Type comparisons like type(x) is SomeClass
and
type(x) == SomeClass
are converted to isinstance(x, SomeClass)
.
while 1
becomes while True
. This fixer also tries to make use of
sorted()
in appropriate places. For example, this block
L = list(some_iterable)
L.sort()
is changed to
L = sorted(some_iterable)
import
imports
imports2
imports
fixer only because of technical limitations.input
input(prompt)
to eval(input(prompt))
.intern
intern()
to sys.intern()
.isinstance
isinstance()
. For example, isinstance(x, (int, int))
is converted to isinstance(x, int)
and isinstance(x, (int, float, int))
is converted to isinstance(x, (int, float))
.itertools_imports
itertools.ifilter()
, itertools.izip()
, and itertools.imap()
. Imports of itertools.ifilterfalse()
are also changed to itertools.filterfalse()
.itertools
itertools.ifilter()
, itertools.izip()
, and itertools.imap()
to their built-in equivalents. itertools.ifilterfalse()
is changed to itertools.filterfalse()
.long
long
to int
.map
map()
in a list
call. It also changes map(None, x)
to list(x)
. Using from future_builtins import map
disables this fixer.metaclass
__metaclass__ = Meta
in the class body) to the new (class X(metaclass=Meta)
).methodattrs
meth.im_func
is converted to meth.__func__
.ne
<>
, to !=
.next
next()
methods to the next()
function. It also renames next()
methods to __next__()
.nonzero
__nonzero__()
to __bool__()
.numliterals
operator
Converts calls to various functions in the operator
module to other,
but equivalent, function calls. When needed, the appropriate import
statements are added, e.g. import collections.abc
. The following mapping
are made:
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paren
[x for x in 1, 2]
becomes [x for x in (1, 2)]
.print
print
statement to the print()
function.raise
raise E, V
to raise E(V)
, and raise E, V, T
to raise E(V).with_traceback(T)
. If E
is a tuple, the translation will be incorrect because substituting tuples for exceptions has been removed in 3.0.raw_input
raw_input()
to input()
.reduce
reduce()
to functools.reduce()
.reload
reload()
to importlib.reload()
.renames
sys.maxint
to sys.maxsize
.repr
repr()
function.set_literal
set
constructor with set literals. This fixer is optional.standarderror
StandardError
to Exception
.sys_exc
sys.exc_value
, sys.exc_type
, sys.exc_traceback
to use sys.exc_info()
.throw
throw()
method.tuple_params
types
types
module.unicode
unicode
to str
.ws_comma
xreadlines
for x in file.xreadlines()
to for x in file
.zip
zip()
usage in a list
call. This is disabled when from future_builtins import zip
appears.