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.
Mypy requires typed-ast in order to check Python 2 code. You can install it
pip install 'mypy[python2]'.
Run mypy in Python 2 mode by using the
$ 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
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
clsvariable of methods.
- Docstring always comes after the type comment.
**kwargsthe type should be prefixed with
**, respectively (except when using the multi-line annotation syntax described below). Again, the above example illustrates this.
- Things like
Anymust be imported from
typing, even if they are only used in comments.
- In Python 2 mode
stris implicitly promoted to
unicode, similar to how
intis compatible with
float. This is unlike
strin Python 3, which are incompatible.
bytesin 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):
def send_email(address, # type: Union[str, List[str]] sender, # type: str cc, # type: Optional[List[str]] bcc, # type: Optional[List[str]] subject='', body=None # type: List[str] ): # type: (...) -> bool """Send an email message. Return True if successful.""" <code>
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
When using multi-line comments, you do not need to prefix the
types of your
**kwarg parameters with
For example, here is how you would annotate the first example using
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.""" ...
- You should include types for arguments with default values in the
methodin 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.
- Variables use a comment-based type syntax (explained in Explicit types for variables).
- You don’t need to use string literal escapes for forward references within comments (string literal escapes are explained later).
- Mypy uses a separate set of library stub files in typeshed for Python 2. Library support may vary between Python 2 and Python 3.