2to3 is a Python program that reads Python 2.x source code and applies a seriesof fixers to transform it into valid Python 3.x code. The standard librarycontains a rich set of fixers that will handle almost all code. 2to3 supportinglibrary
lib2to3 is, however, a flexible and generic library, so it ispossible to write your own fixers for 2to3. lib2to3 could also beadapted to custom applications in which Python code needs to be editedautomatically.
25.4.1. Using 2to3¶
A Python script with the extension.spec, this file includes details about how your Python app needs to be packed up. The first time you run PyInstaller on your app, PyInstaller will generate a.
2to3 will usually be installed with the Python interpreter as a script. It isalso located in the
Tools/scripts directory of the Python root.
2to3’s basic arguments are a list of files or directories to transform. Thedirectories are recursively traversed for Python sources.
Here is a sample Python 2.x source file,
example.py :
It can be converted to Python 3.x code via 2to3 on the command line:
A diff against the original source file is printed. 2to3 can also write theneeded modifications right back to the source file. (A backup of the originalfile is made unless
-n is also given.) Writing the changes back isenabled with the -w flag:
After transformation,
example.py looks like this:
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 runcan be given with -f . Likewise the -x explicitly disables afixer. The following example runs only the imports and has_key fixers:
This command runs every fixer except the
apply fixer:
Some fixers are explicit, meaning they aren’t run by default and must belisted on the command line to be run. Here, in addition to the default fixers,the
idioms fixer is run:
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 warningbeneath the diff for a file. You should address the warning in order to havecompliant 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 requirethe module to be valid Python. For example, doctest like examples in a reSTdocument could also be refactored with this option.
The
-v option enables output of more information on the translationprocess.
Since some print statements can be parsed as function calls or statements, 2to3cannot always read files containing the print function. When 2to3 detects thepresence of the
from__future__importprint_function compiler directive, itmodifies its internal grammar to interpret print() as a function. Thischange can also be enabled manually with the -p flag. Use-p to run fixers on code that already has had its print statementsconverted.
The
-o or --output-dir option allows specification of analternate directory for processed output files to be written to. The-n flag is required when using this as backup files do not make sensewhen not overwriting the input files.
New in version 2.7.3: The
-o option was added.
The
-W or --write-unchanged-files flag tells 2to3 to alwayswrite output files even if no changes were required to the file. This is mostuseful with -o so that an entire Python source tree is copied withtranslation from one directory to another.This option implies the -w flag as it would not make sense otherwise.
The
--add-suffix option specifies a string to append to all outputfilenames. The -n flag is required when specifying this as backupsare not necessary when writing to different filenames. Example:
Will cause a converted file named
example.py3 to be written.
New in version 2.7.3: The
--add-suffix option was added.
To translate an entire project from one directory tree to another use:
25.4.2. Fixers¶
Each step of transforming code is encapsulated in a fixer. The command
2to3-l lists them. As documented above, each can be turned onand off individually. They are described here in more detail.
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