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 Python 2 module search path. Use pip install typing to install the module. This also works for Python 3 versions prior to 3.5 that don’t include typing in the standard library.

The example below illustrates the Python 2 function type annotation syntax. This syntax 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
        """Docstring comes after type comment."""

It’s worth going through these details carefully to avoid surprises:

  • You don’t provide an annotation for the self / cls variable of methods.
  • Docstring always comes after the type comment.
  • For *args and **kwargs the type should be prefixed with * or **, respectively (except when using the multi-line annotation syntax described below). Again, the above example illustrates this.
  • Things like Any must be imported from typing, even if they are only used in comments.
  • 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. (This might change in the future.)

Multi-line Python 2 function annotations

Mypy also supports a multi-line comment annotation syntax. You can provide a separate annotation for each argument using the variable annotation syntax. When using the single-line annotation syntax described above, functions with long argument lists tend to result in overly long type comments and it’s often tricky to see which argument type corresponds to which argument. The alternative, multi-line annotation syntax makes long annotations easier to read and write.

Here is an example (from PEP 484):

               sender,      # type: str
               cc,          # type: Optional[List[str]]
               bcc,         # type: Optional[List[str]]
               body=None    # type: List[str]
    # type: (...) -> bool

You write a separate annotation for each function argument on the same line as the argument. Each annotation must be on a separate line. If you leave out an annotation for an argument, it defaults to Any. You provide a return type annotation in the body of the function using the form # type: (...) -> rt, where rt is the return type. Note that the return type annotation contains literal three dots.

Note that when using multi-line comments, you do not need to prefix the types of your *arg and **kwarg parameters with * or **. For example, here is how you would annotate the first example using multi-line comments.

from typing import List

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

Additional notes

  • You should include types for arguments with default values in the annotation. The opt argument of method in the example at the beginning of this section is an example of this.
  • The annotation can be on the same line as the function header or on the following line.
  • The type syntax for variables is the same as for Python 3.
  • 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.