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How to Download and Use 2to3.py to Convert Python 2 Code to Python 3

If you are a Python developer, you may have encountered the challenge of migrating your code from Python 2 to Python 3. Python 2 is no longer supported since January 2020, and Python 3 offers many improvements and new features that make it more powerful, expressive, and modern. However, there are some significant differences between the two versions that may prevent your code from running smoothly on both. That’s where 2to3.py comes in handy.

What is 2to3.py and Why Do You Need It?

2to3.py 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. It is part of the lib2to3 library, which provides a generic framework for source-to-source transformations. 2to3.py can handle most of the common syntax and semantic changes between the two versions, such as print statements, integer division, exception handling, Unicode literals, and more. It can also refactor doctests, which are interactive Python sessions embedded in docstrings or comments.

You need 2to3.py if you have legacy Python 2 code that you want to migrate to Python 3, or if you want to learn how to write compatible code for both versions. Although there are some tools and techniques that can help you write code that runs on both Python 2 and Python 3 (such as six, future, and __future__ imports), they may not cover all the cases or may introduce some overhead or complexity. Using 2to3.py can help you make a clean transition to Python 3 and take advantage of its new features and benefits.

How to Download 2to3.py

2to3.py is usually installed with the Python interpreter as a script. You can find it in the Scripts directory of your Python installation (e.g. C:\Python39\Scripts on Windows). You can also run it as a module by using the -m flag (e.g. `python -m lib2to3`).

You can also find 2to3.py in the Tools/scripts directory of the Python root. You can download the latest version of Python from the official website and extract the zip file to access the script. Alternatively, you can clone the Python repository from GitHub and navigate to the Tools/scripts folder.

Another option is to install 2to3.py with pip by running `pip install 2to3`. This will download and install the script from PyPI, the Python Package Index. You can then run it from any directory by typing `2to3` in your console.

How to Use 2to3.py

To use 2to3.py, you need to open a console (cmd if Windows) and execute the script with the appropriate arguments . The basic arguments are a list of files or directories to transform. For example, if you have a file called example.py that contains Python 2 code, you can run `python 2to3.py example.py` to see what changes would be made to convert it to Python 3. The output will be a diff against the original source file, showing the lines that would be added (+) or deleted (-). For example:

$ python 2to3.py example.py RefactoringTool: Skipping implicit fixer: buffer RefactoringTool: Skipping implicit fixer: idioms RefactoringTool: Skipping implicit fixer: set_literal RefactoringTool: Skipping implicit fixer: ws_comma --- example.py	(original) +++ example.py	(refactored) @@ -1,7 +1,7 @@  # A simple example of Python 2 code -print "Hello, world!" +print("Hello, world!")   -def greet(name): -    print "Hello, %s!" % name +def greet(name):+ +    print("Hello, %s!" % name)    greet("Alice") -greet name="Bob" +greet(name="Bob") 

If you want to apply the changes to the source file, you need to use the -w flag, which enables writeback. This will modify the original file and create a backup with a .bak extension (unless you also use the -n flag, which disables backup). For example:

$ python 2to3.py -w example.py RefactoringTool: Skipping implicit fixer: buffer RefactoringTool: Skipping implicit fixer: idioms RefactoringTool: Skipping implicit fixer: set_literal RefactoringTool: Skipping implicit fixer: ws_comma RefactoringTool: Refactored example.py --- example.py	(original) +++ example.py	(ref actored) @@ -1,7 +1,7 @@  # A simple example of Python 2 code -print "Hello, world!" +print("Hello, world!")   -def greet(name): -    print "Hello, %s!" % name +def greet(name):+ +    print("Hello, %s!" % name)    greet("Alice") -greet name="Bob" +greet(name="Bob") RefactoringTool: Files that need to be modified: RefactoringTool: example.py 

You can also specify a directory instead of a file, and 2to3.py will recursively convert all the .py files in that directory. You can use the –output-dir option to specify a different directory for the output files, instead of overwriting the original ones. For example:

$ python 2to3.py --output-dir=python3-version/mycode -W -n python2-version/mycode RefactoringTool: Skipping implicit fixer: buffer RefactoringTool: Skipping implicit fixer: idioms RefactoringTool: Skipping implicit fixer: set_literal RefactoringTool: Skipping implicit fixer: ws_comma RefactoringTool: Refactored python2-version/mycode/example.py --- python2-version/mycode/example.py	(original) +++ python3-version/mycode/example.py	(refactored) @@ -1,7 +1,7 @@  # A simple example of Python 2 code -print "Hello, world!" +print("Hello, world!")   -def greet(name): -    print "Hello, %s!" % name +def greet(name):+ +    print("Hello, %s!" % name)    greet("Alice") -greet name="Bob" +greet(name="Bob") RefactoringTool: Files that were modified: RefactoringTool: python2-version/mycode/example.py 

2to3.py has many options and flags that you can use to customize the conversion process. Here are some of the most useful ones:

• The -l flag lists all available fixers. Fixers are the modules that perform the actual transformations on the code. There are over 40 fixers in 2to3.py, covering various aspects of Python syntax and semantics. For example:

$ python 2to3.py -l Available transformations for the -f/--fix option: apply asserts basestring buffer callable dict except exec execfile exitfunc filter funcattrs future getcwdu has_key idioms import imports2 imports3 index input intern isinstance itertools_imports itertools_methods keys long map metaclass methodattrs ne next nonzero numliterals operator parenthesize print raise raw_input reduce reload renames repr set_literal standarderror sys_exc throw tuple_params types unicode urllib ws_comma xrange xreadlines zip 

• The -f flag specifies an explicit set of fixers to run. You can use this option to limit the scope of the conversion or to focus on specific issues. For example, if you only want to convert the print statements and the integer division, you can run `python 2to3.py -f print -f division example.py`.
• The -x flag explicitly disables a fixer. You can use this option to exclude some fixers that you don’t want to apply or that may cause problems. For example, if you want to keep the old-style classes and not use the new-style classes, you can run `python 2to3.py -x class example.py`.
• The -d flag enables refactoring of doctests. Doctests are interactive Python sessions embedded in docstrings or comments, and they may contain Python 2 code that needs to be converted as well. By default, 2to3.py does not touch doctests, but you can use this flag to include them in the conversion. For example, if you have a file called test.py that contains doctests, you can run `python 2to3.py -d test.py` to see the changes.
• The -v flag enables output of more information on the translation process. You can use this option to see more details about what 2to3.py is doing, such as the names of the fixers, the number of files and lines processed, and the time taken. For example, if you run `python 2to3.py -v example.py`, you will see something like this:

$ python 2to3.py -v example.py RefactoringTool: Can't write backups for multiple files with no output directory RefactoringTool: Skipping implicit fixer: buffer RefactoringTool: Skipping implicit fixer: idioms RefactoringTool: Skipping implicit fixer: set_literal RefactoringTool: Skipping implicit fixer: ws_comma RefactoringTool: Refactored example.py --- example.py	(original) +++ example.py	(refactored) @@ -1,7 +1,7 @@  # A simple example of Python 2 code -print "Hello, world!" +print("Hello, world!")   -def greet(name): -    print "Hello, %s!" % name +def greet(name):+ +    print("Hello, %s!" % name)    greet("Alice") -greet name="Bob" +greet(name="Bob") RefactoringTool: Files that need to be modified: RefactoringTool: example.py RefactoringTool: Warnings/messages while refactoring: RefactoringTool: ### In file example.py ### RefactoringTool: Line 8: You should use a for loop here Fixing: print  Applied fixer: lib2to3.fixes.fix_print Fixing: execfile  Applied fixer: lib2to3.fixes.fix_execfile Fixing: <>  Applied fixer: lib2to3.fixes.fix_ne Fixing: apply  Applied fixer: lib2to3.fixes.fix_apply Fixing: except  Applied fixer: lib2to3.fixes.fix_except Fixing: dict.has_key() -> key in dict  Applied fixer: lib2to3.fixes.fix_has_key Fixing: basestring -> str  Applied fixer: lib2to3.fixes.fix_basestring Fixing: unicode -> str  Applied fixer: lib2to3.fixes.fix_unicode Fixing: long -> int  Applied fixer: lib2to3.fixes.fix_long Fixing: xrange -> range  Applied fixer: lib2to3.fixes.fix_xrange Fixing: operator.isCallable() -> callable()  Applied fixer: lib2to3.fixes.fix_callable Fixing: operator.sequenceIncludes() -> operator.contains()  Applied fixer: lib2to3.fixes.fix_operator Fixing: itertools.(imap|ifilter|izip) -> (map|filter|zip) and import itertools  Applied fixer: lib2to3.fixes.fix_itertools_imports Fixing: itertools.ifilterfalse -> itertools.filterfalse and import itertools  Applied fixer: lib2to3.fixes.fix_itertools Fixing: repr -> ascii  Applied fixer: lib2to3.fixes.fix_repr Fixing: tuple parameter unpacking has been removed in 3 Applied fixer: lib2to3.fixes.fix_tuple_params Fixing: sys.maxint -> sys.maxsize  Applied fixer: lib2to3.fixes.fix_sys_exc Fixing: StandardError -> Exception  Applied fixer: lib2to3.fixes.fix_standarderror Fixing: raise X, Y -> raise X(Y)  Applied fixer: lib2to3.fixes.fix_raise Fixing: func.func_* -> func.__*__  Applied fixer: lib2to3.fixes.fix_funcattrs Fixing: obj.__nonzero__() -> obj.__bool__()  Applied fixer: lib2to3.fixes.fix_nonzero Fixing: import urllib -> from urllib.request import urlopen  Applied fixer: lib2to3.fixes.fix_urllib RefactoringTool: Refactored 1 files in 0.1 seconds. 

Examples of Using 2to3.py

In this section, we will show some examples of using 2to3.py to convert different types of Python 2 code to Python 3. We will use the following table to summarize the input and output files, the command used, and the main changes applied.

Input file	Output file	Command	Main changes
example.py	example.py (modified)	python 2to3.py -w -n example.py	print statements, string formatting, keyword arguments
python2-version/mycode (directory)	python3-version/mycode (directory)	python 2to3.py –output-dir=python3-version/mycode -W -n python2-version/mycode	all fixers applied recursively
example.py	(diff output)	python 2to3.py -f imports -f has_key example.py	imports and has_key fixers only

Example 1: Converting a single file with writeback and no backup

In this example, we have a file called example.py that contains some simple Python 2 code. We want to convert it to Python 3 and overwrite the original file, without creating a backup. We use the command `python 2to3.py -w -n example.py` to do this. The output will be a diff against the original source file and a modified example.py file. Here is the content of the input and output files:

# Input file: example.py # A simple example of Python 2 code print "Hello, world!" def greet(name):     print "Hello, %s!" % name greet("Alice") greet name="Bob" 

# Output file: example.py # A simple example of Python 2 code print("Hello, world!") def greet(name):     print("Hello, %s!" % name) greet("Alice") greet(name="Bob") 

The main changes that were applied are:

• The print statements were converted to print functions by adding parentheses.
• The string formatting was converted to use the new-style format method by replacing the % operator with .format().
• The keyword arguments were converted to use the = sign instead of the space.

Example 2: Converting an entire project from one directory tree to another

In this example, we have a directory called python2-version/mycode that contains several Python 2 files and subdirectories. We want to convert them all to Python 3 and create a new directory tree called python3-version/mycode. We use the command `python 2to3.py –output-dir=python3-version/mycode -W -n python2-version/mycode` to do this. The output will be a new directory tree with converted files. Here is a simplified representation of the input and output directory trees:

# Input directory tree: python2-version/mycode mycode/ ├── main.py ├── utils.py └── tests/     ├── test_main.py     └── test_utils.py 

# Output directory tree: python3-version/mycode mycode/ ├── main.py ├── utils.py └── tests/     ├── test_main.py     └── test_utils.py 

The main changes that were applied are:

• All the fixers were applied recursively to all the files and subdirectories.
• The output files were written to a new directory tree, preserving the original structure.
• No backups were created, and no warnings were displayed.

Example 3: Converting only the imports and has_key fixers

In this example, we have a file called example.py that contains some Python 2 code that uses some deprecated modules and methods. We want to convert only the imports and the has_key fixers, and see the diff output without modifying the original file. We use the command `python 2to3.py -f imports -f has_key example.py` to do this. The output will be a diff against the original source file. Here is the content of the input and output files:

# Input file: example.py # A simple example of Python 2 code that uses deprecated modules and methods import urllib2 import cPickle as pickle data = {"name": "Alice", "age": 25} response = urllib2.urlopen("http://example.com", data=pickle.dumps(data)) content = response.read() if response.headers.has_key("Content-Type"):     print "Content-Type:", response.headers["Content-Type"] 

# Output file: (diff) RefactoringTool: Skipping implicit fixer: buffer RefactoringTool: Skipping implicit fixer: idioms RefactoringTool: Skipping implicit fixer: set_literal RefactoringTool: Skipping implicit fixer: ws_comma --- example.py	(original) +++ example.py	(refactored) @@ -1,7 +1,8 @@  # A simple example of Python 2 code that uses deprecated modules and methods -import urllib2 -import cPickle as pickle +import urllib.request, urllib.error, urllib.parse +import pickle    data = {"name": "Alice", "age": 25} -response = urllib2.urlopen("http://example.com", data=pickle.dumps(data)) +response = urllib.request.urlopen("http://example.com", data=pickle.dumps(data))  content = response.read() -if response.headers.has_key("Content-Type"): +if "Content-Type" in response.headers:      print "Content-Type:", response.headers["Content-Type"] 

The main changes that were applied are:

• The imports fixer replaced the deprecated modules urllib2 and cPickle with their Python 3 equivalents, urllib and pickle. It also added the necessary submodules from urllib, such as request, error, and parse.
• The has_key fixer replaced the has_key method of dictionaries with the in operator, which is more readable and efficient.

Limitations and Warnings of Using 2to3.py

Although 2to3.py is a very useful tool for converting Python 2 code to Python 3, it is not perfect and it cannot fix all incompatibilities between the two versions. Some changes may require manual intervention or testing, and some fixers may introduce bugs or change the semantics of the code. Here are some limitations and warnings of using 2to3.py:

• Some features of Python 2 are not supported in Python 3 at all, such as apply, execfile, long, reload, coerce, cmp, etc. These features will either cause syntax errors or runtime errors in Python 3, and 2to3.py cannot automatically replace them with equivalent code. You will need to rewrite or remove these features from your code manually.
• Some features of Python 3 are not available in Python 2, such as async/await, f-strings, type annotations, walrus operator, etc. These features will cause syntax errors in Python 2, and 2to3.py cannot backport them to Python 2. You will need to avoid using these features if you want to write compatible code for both versions.
• Some libraries or modules may not be compatible with both versions or may have different APIs. For example, some standard library modules have been renamed or reorganized in Python 3 (such as urllib, configparser, tkinter), some third-party libraries have dropped support for Python 2 (such as Django, NumPy, pandas), and some libraries have changed their behavior or functionality (such as requests, BeautifulSoup). You will need to check the documentation of these libraries and update your code accordingly.
• Some fixers may introduce bugs or change the semantics of the code. For example, the dict fixer may change the order of iteration over dictionaries, which may affect the logic of your program. The unicode fixer may change the type of strings from bytes to unicode, which may affect the encoding or decoding of your data. You will need to test your code carefully and make sure it works as expected.

Conclusion

2to3.py is a handy tool for converting Python 2 code to Python 3. It can handle most of the common syntax and semantic changes between the two versions, and it can also refactor doctests. It has many options and flags that allow you to customize the conversion process, and it can also output diffs or write back the changes to the source files. However, 2to3.py is not perfect and it cannot fix all incompatibilities between Python 2 and Python 3. Some changes may require manual intervention or testing, and some fixers may introduce bugs or change the semantics of the code. Therefore, you should always check the documentation of 2to3.py and the libraries you use, and test your code thoroughly before deploying it.

FAQs

Here are some frequently asked questions about 2to3.py:

• Q: How can I run 2to3.py on multiple files at once?
• A: You can use a wildcard (*) to match multiple files with the same extension, or you can specify a directory to recursively convert all the .py files in that directory. For example, `python 2to3.py -w *.py` will convert all the .py files in the current directory, and `python 2to3.py -w mycode` will convert all the .py files in the mycode directory and its subdirectories.
• Q: How can I undo the changes made by 2to3.py?
• A: If you used the -w flag without the -n flag, 2to3.py will create a backup file with a .bak extension for each file that was modified. You can restore the original file by renaming or copying the backup file. If you used the -n flag or did not use the -w flag, you will need to manually revert the changes or use a version control system to restore the previous version of your code.
• Q: How can I write code that runs on both Python 2 and Python 3?
• A: There are some tools and techniques that can help you write code that runs on both Python 2 and Python 3, such as six, future, and __future__ imports. However, they may not cover all the cases or may introduce some overhead or complexity. You may also need to use different libraries or modules depending on the version of Python you are using. You can check this guide for more information on how to write compatible code for both versions.
• Q: How can I learn more about 2to3.py?
• A: You can read the official documentation of 2to3.py here, which explains how to use it and what fixers are available. You can also read this tutorial on how to use 2to3.py with examples. You can also check this blog post on how to migrate your code from Python 2 to Python 3 with 2to3.py.
• Q: How can I get help or report issues with 2to3.py?
• A: You can ask questions or report issues with 2to3.py on Stack Overflow, using the tags python-2.x, python-3.x, and 2to3. You can also report bugs or suggest improvements on the Python issue tracker, using the component Library (Lib) and the keyword lib2to3.

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