Is it possible to type hint a lambda function?
You can, sort of, in Python 3.6 and up using PEP 526 variable annotations. You can annotate the variable you assign the lambda
result to with the typing.Callable
generic:
from typing import Callable
func: Callable[[str, str], int] = lambda var1, var2: var1.index(var2)
This doesn't attach the type hinting information to the function object itself, only to the namespace you stored the object in, but this is usually all you need for type hinting purposes.
However, you may as well just use a function statement instead; the only advantage that a lambda
offers is that you can put a function definition for a simple expression inside a larger expression. But the above lambda is not part of a larger expression, it is only ever part of an assignment statement, binding it to a name. That's exactly what a def func(var1: str, var2: str): return var1.index(var2)
statement would achieve.
Note that you can't annotate *args
or **kwargs
arguments separately either, as the documentation for Callable
states:
There is no syntax to indicate optional or keyword arguments; such function types are rarely used as callback types.
That limitation does not apply to a PEP 544 protocol with a __call__
method; use this if you need a expressive definition of what arguments should be accepted. You need Python 3.8 or install the typing-extensions
project for a backport:
from typing_extensions import Protocol
class SomeCallableConvention(Protocol):
def __call__(self, var1: str, var2: str, spam: str = "ham") -> int:
...
func: SomeCallableConvention = lambda var1, var2, spam="ham": var1.index(var2) * spam
For the lambda
expression itself, you can't use any annotations (the syntax on which Python's type hinting is built). The syntax is only available for def
function statements.
From PEP 3107 - Function Annotations:
lambda 's syntax does not support annotations. The syntax of lambda could be changed to support annotations, by requiring parentheses around the parameter list. However it was decided not to make this change because:
- It would be an incompatible change.
- Lambda's are neutered anyway.
- The lambda can always be changed to a function.
You can still attach the annotations directly to the object, the function.__annotations__
attribute is a writable dictionary:
>>> def func(var1: str, var2: str) -> int:
... return var1.index(var2)
...
>>> func.__annotations__
{'var1': <class 'str'>, 'return': <class 'int'>, 'var2': <class 'str'>}
>>> lfunc = lambda var1, var2: var1.index(var2)
>>> lfunc.__annotations__
{}
>>> lfunc.__annotations__['var1'] = str
>>> lfunc.__annotations__['var2'] = str
>>> lfunc.__annotations__['return'] = int
>>> lfunc.__annotations__
{'var1': <class 'str'>, 'return': <class 'int'>, 'var2': <class 'str'>}
Not that dynamic annotations like these are going to help you when you wanted to run a static analyser over your type hints, of course.
Since Python 3.6, you can (see PEP 526):
from typing import Callable
is_even: Callable[[int], bool] = lambda x: (x % 2 == 0)