python - Is it possible to type hint a lambda function?

Question

Currently, in Python, a function's parameters and return types can be type hinted as follows:

def func(var1: str, var2: str) -> int:
    return var1.index(var2)

Which indicates that the function takes two strings, and returns an integer.

However, this syntax is highly confusing with lambdas, which look like:

func = lambda var1, var2: var1.index(var2)

I've tried putting in type hints on both parameters and return types, and I can't figure out a way that doesn't cause a syntax error.

Is it possible to type hint a lambda function? If not, are there plans for type hinting lambdas, or any reason (besides the obvious syntax conflict) why not?

I think that the usual use-case for a lambda is nested within another function (contrasted with regular functions which are defined in one place and called somewhere entirely different). If you already know the types in that function, then (in principle), it should be pretty easy to figure out what types that lambda is going to receive. Additionally, modifying the syntax to support annotations would just make your lambda harder to read. — mgilson
Why would you want to do that? The lambda syntax is for "throw-away" functions for use within very constrained contexts, for example, as the key argument to the sorted built-in. I really do not see a point in adding type hints in such limited contexts. In addition, using PEP-526 variable annotations to add type hints to lambda completely misses the point, IMHO. The lambda syntax is intended to define anonymous functions. What's the point of using lambda and immediately bind it to a variable? Just use def! — Luciano Ramalho

Martijn Pieters Martijn Pieters · Accepted Answer · 2015-11-20T18:46:11

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.

python - Is it possible to type hint a lambda function?

3 Answers