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# 26.4. 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.

## 26.4.1. 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 to 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.

When the -p is passed, 2to3 treats `print` as a function instead of a statement. This is useful when `from __future__ import print_function` is being used. If this option is not given, the print fixer will surround print calls in an extra set of parentheses because it cannot differentiate between the print statement with parentheses (such as `print ("a" + "b" + "c")`) and a true function call.

## 26.4.2. `lib2to3` - 2to3’s library

Warning

The `lib2to3` API should be considered unstable and may change drastically in the future.

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