| Python 3.6.5 Documentation > 2to3 - Automated Python 2 to 3 code translation
2to3 - Automated Python 2 to 3 code translation
***********************************************
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.
Using 2to3
==========
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
Fixers
======
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
Removes usage of "apply()". For example "apply(function, *args,
**kwargs)" is converted to "function(*args, **kwargs)".
asserts
Replaces deprecated "unittest" method names with the correct ones.
+----------------------------------+--------------------------------------------+
| From | To |
+==================================+============================================+
| "failUnlessEqual(a, b)" | "assertEqual(a, b)" |
+----------------------------------+--------------------------------------------+
| "assertEquals(a, b)" | "assertEqual(a, b)" |
+----------------------------------+--------------------------------------------+
| "failIfEqual(a, b)" | "assertNotEqual(a, b)" |
+----------------------------------+--------------------------------------------+
| "assertNotEquals(a, b)" | "assertNotEqual(a, b)" |
+----------------------------------+--------------------------------------------+
| "failUnless(a)" | "assertTrue(a)" |
+----------------------------------+--------------------------------------------+
| "assert_(a)" | "assertTrue(a)" |
+----------------------------------+--------------------------------------------+
| "failIf(a)" | "assertFalse(a)" |
+----------------------------------+--------------------------------------------+
| "failUnlessRaises(exc, cal)" | "assertRaises(exc, cal)" |
+----------------------------------+--------------------------------------------+
| "failUnlessAlmostEqual(a, b)" | "assertAlmostEqual(a, b)" |
+----------------------------------+--------------------------------------------+
| "assertAlmostEquals(a, b)" | "assertAlmostEqual(a, b)" |
+----------------------------------+--------------------------------------------+
| "failIfAlmostEqual(a, b)" | "assertNotAlmostEqual(a, b)" |
+----------------------------------+--------------------------------------------+
| "assertNotAlmostEquals(a, b)" | "assertNotAlmostEqual(a, b)" |
+----------------------------------+--------------------------------------------+
basestring
Converts "basestring" to "str".
buffer
Converts "buffer" to "memoryview". This fixer is optional because
the "memoryview" API is similar but not exactly the same as that of
"buffer".
dict
Fixes dictionary iteration methods. "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
Converts "except X, T" to "except X as T".
exec
Converts the "exec" statement to the "exec()" function.
execfile
Removes usage of "execfile()". The argument to "execfile()" is
wrapped in calls to "open()", "compile()", and "exec()".
exitfunc
Changes assignment of "sys.exitfunc" to use of the "atexit" module.
filter
Wraps "filter()" usage in a "list" call.
funcattrs
Fixes function attributes that have been renamed. For example,
"my_function.func_closure" is converted to
"my_function.__closure__".
future
Removes "from __future__ import new_feature" statements.
getcwdu
Renames "os.getcwdu()" to "os.getcwd()".
has_key
Changes "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
Detects sibling imports and converts them to relative imports.
imports
Handles module renames in the standard library.
imports2
Handles other modules renames in the standard library. It is
separate from the "imports" fixer only because of technical
limitations.
input
Converts "input(prompt)" to "eval(input(prompt))".
intern
Converts "intern()" to "sys.intern()".
isinstance
Fixes duplicate types in the second argument of "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
Removes imports of "itertools.ifilter()", "itertools.izip()", and
"itertools.imap()". Imports of "itertools.ifilterfalse()" are also
changed to "itertools.filterfalse()".
itertools
Changes usage of "itertools.ifilter()", "itertools.izip()", and
"itertools.imap()" to their built-in equivalents.
"itertools.ifilterfalse()" is changed to "itertools.filterfalse()".
long
Renames "long" to "int".
map
Wraps "map()" in a "list" call. It also changes "map(None, x)" to
"list(x)". Using "from future_builtins import map" disables this
fixer.
metaclass
Converts the old metaclass syntax ("__metaclass__ = Meta" in the
class body) to the new ("class X(metaclass=Meta)").
methodattrs
Fixes old method attribute names. For example, "meth.im_func" is
converted to "meth.__func__".
ne
Converts the old not-equal syntax, "<>", to "!=".
next
Converts the use of iterator’s "next()" methods to the "next()"
function. It also renames "next()" methods to "__next__()".
nonzero
Renames "__nonzero__()" to "__bool__()".
numliterals
Converts octal literals into the new syntax.
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". The following mapping are made:
+------------------------------------+--------------------------------------------+
| From | To |
+====================================+============================================+
| "operator.isCallable(obj)" | "hasattr(obj, '__call__')" |
+------------------------------------+--------------------------------------------+
| "operator.sequenceIncludes(obj)" | "operator.contains(obj)" |
+------------------------------------+--------------------------------------------+
| "operator.isSequenceType(obj)" | "isinstance(obj, collections.Sequence)" |
+------------------------------------+--------------------------------------------+
| "operator.isMappingType(obj)" | "isinstance(obj, collections.Mapping)" |
+------------------------------------+--------------------------------------------+
| "operator.isNumberType(obj)" | "isinstance(obj, numbers.Number)" |
+------------------------------------+--------------------------------------------+
| "operator.repeat(obj, n)" | "operator.mul(obj, n)" |
+------------------------------------+--------------------------------------------+
| "operator.irepeat(obj, n)" | "operator.imul(obj, n)" |
+------------------------------------+--------------------------------------------+
paren
Add extra parenthesis where they are required in list
comprehensions. For example, "[x for x in 1, 2]" becomes "[x for x
in (1, 2)]".
print
Converts the "print" statement to the "print()" function.
raise
Converts "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
Converts "raw_input()" to "input()".
reduce
Handles the move of "reduce()" to "functools.reduce()".
reload
Converts "reload()" to "imp.reload()".
renames
Changes "sys.maxint" to "sys.maxsize".
repr
Replaces backtick repr with the "repr()" function.
set_literal
Replaces use of the "set" constructor with set literals. This
fixer is optional.
standarderror
Renames "StandardError" to "Exception".
sys_exc
Changes the deprecated "sys.exc_value", "sys.exc_type",
"sys.exc_traceback" to use "sys.exc_info()".
throw
Fixes the API change in generator’s "throw()" method.
tuple_params
Removes implicit tuple parameter unpacking. This fixer inserts
temporary variables.
types
Fixes code broken from the removal of some members in the "types"
module.
unicode
Renames "unicode" to "str".
urllib
Handles the rename of "urllib" and "urllib2" to the "urllib"
package.
ws_comma
Removes excess whitespace from comma separated items. This fixer
is optional.
xrange
Renames "xrange()" to "range()" and wraps existing "range()" calls
with "list".
xreadlines
Changes "for x in file.xreadlines()" to "for x in file".
zip
Wraps "zip()" usage in a "list" call. This is disabled when "from
future_builtins import zip" appears.
"lib2to3" - 2to3’s library
==========================
**Source code:** Lib/lib2to3/
======================================================================
Note: The "lib2to3" API should be considered unstable and may change
drastically in the future.
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