How to restrict a parameter of python function must be string or a function like lambda expression

Question

For example, the following, the first parameter should be restricted to a string, and the second parameter should be a function. However, this is wrong syntax for both. Anyone can help suggest the correct syntax to impose the type restriction?

def GetArg(msg:String, converter:lambda, default):
    print("{}, the default is '{}':".format(msg, default))
    return converter(stdin.readline().strip())

It gives error

Traceback (most recent call last):
  File "E:/stdin_ext.py", line 4, in <module>
    def GetArg(msg:String, converter, default:String):
NameError: name 'String' is not defined

and

  File "E:/stdin_ext.py", line 4
    def GetArg(msg:String, converter:lambda, default:String):
                                       ^
SyntaxError: invalid syntax

modesitt · Accepted Answer · 2018-08-02 14:42:25Z

2

You can use the typing module.

from typing import Callable

def get_arg(msg: str, converter: Callable[[str], str], default) -> str:
    print(f"{msg}, the default is '{default}':")
    return converter(stdin.readline().strip())

assuming your function converter takes a string a returns a string, and that get_arg returns a string. Note the code has also been modified to follow python's naming convention and uses f strings over older style format strings.

Also note that these type hints are just that, hints. They are not checked statically or at runtime. Although, certain IDE's will help you ensure they are correct.

edited Aug 2, 2018 at 14:42

answered Aug 2, 2018 at 14:36

modesitt

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Comments

Luca Cappelletti · Accepted Answer · 2018-08-02 14:41:26Z

0

You should use str instead of String and Callable instead of lambda:

from typing import Callable    

def GetArg(msg:str, converter:Callable, default):
    print("{}, the default is '{}':".format(msg, default))
    return converter(stdin.readline().strip())

That is, unless you have a specific class called String and you are expecting an argument of its type.

When you write lambda you are defining a new lambda, not specifying a type for a function, whence the error.

I believe it is also important to point out that types in python are only a useful notation but it does not raise any error by itself, for example I could still call GetArg(1,2,3) and don't get any error just by the type hinting (of course I would get an error trying to pass an argument to an int afterwards).

edited Aug 2, 2018 at 14:41

answered Aug 2, 2018 at 14:34

Luca Cappelletti

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2 Comments

Rach Sharp Over a year ago

Isn't a Callable annotation, e.g. get_next_item: Callable[[], str] a better fit?

modesitt Over a year ago

Callable[[], str] implies that the function takes no arguments and returns a string. This is not the case, as the function clearly takes a string. @RachSharp was just giving an example.

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