python2.rst

Type checking Python 2 code

For code that needs to be Python 2.7 compatible, function type annotations are given in comments, since the function annotation syntax was introduced in Python 3. The comment-based syntax is specified in PEP 484.

Run mypy in Python 2 mode by using the --py2 option:

$ mypy --py2 program.py

To run your program, you must have the typing module in your module search path. Use pip install typing to install the module (this also works for Python 3).

The example below illustrates Python 2 function type annotation syntax. This is also valid in Python 3 mode:

from typing import List

def hello(): # type: () -> None
    print 'hello'

class Example:
    def method(self, lst, opt=0, *args, **kwargs):
        # type: (List[str], int, *str, **bool) -> int
        ...

Here are more specifics:

• You should include types for arguments with default values in the annotation. The opt argument of method above is an example of this.
• For *args and **kwargs the type should be prefixed with * or **, respectively. Again, the above example illustrates this.
• The type syntax for variables is the same as for Python 3.
• Things like Any must be imported from typing, even if they are only used in comments.
• You don't provide an annotation for the self/cls variable of methods.
• The annotation can be on the same line as the function header or on the following line.
• You don't need to use string literal escapes for forward references within comments.
• Mypy uses a separate set of library stub files in typeshed for Python 2. Library support may vary between Python 2 and Python 3.
• In Python 2 mode str is implicitly promoted to unicode, similar to how int is compatible with float. This is unlike bytes and str in Python 3, which are incompatible. bytes in Python 2 is equivalent to str.

Note

Currently there's no support for splitting an annotation to multiple lines. This will likely change in the future. (PEP 484 already defines the syntax to use; we just have to implement it.)