How to define a function that only accepts string

Question

Hello I'm newbie in python and I wanted to know if I can create a function that only accepts certain types of values, in this case string, else error

 parameter that needs to be string
            |
            v
def isfloat(a):
    if a.count('.') > 1:
        return False
    for c in a:
        if c.isnumeric() or c == '.':
            v = True
        else:
            return False
    return v

If you want to know whether a is str,you can use isinstance(a,str). — jizhihaoSAMA
– jizhihaoSAMA, Commented Mar 19, 2020 at 13:24
if not isinstance(argument, str): raise ValueError('argument has to be a string') — Klaus D.
– Klaus D., Commented Mar 19, 2020 at 13:25
Instead of image, Always add your code so that we can test it instead of rewriting it again. — Jayesh Dhandha
– Jayesh Dhandha, Commented Mar 19, 2020 at 13:43

Mark Mishyn · Accepted Answer · 2021-09-19 06:01:47Z

8

In Python 3.5+ you can use typing to annotate your function:

def isfloat(a: str):
    # More code here...

But type annotation doesn't actually check types!

So, it's better to add robust type check with assert statement:

def isfloat(a: str):
    assert isinstance(a, str), 'Strings only!'
    # More code here...

With assert your function will raise AssertationError if a is not a string.

edited Sep 19, 2021 at 6:01

answered Mar 19, 2020 at 13:39

Mark Mishyn

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Comments

Jonathan Reynosa · Accepted Answer · 2020-03-19 13:57:29Z

1

You could enclose your code within an if statement such as the following.

def enumerico(a):
    if (isinstance(a, str)):
        <your code>
    else:
        <throw exception or exit function>

edited Mar 19, 2020 at 13:57

answered Mar 19, 2020 at 13:41

Jonathan Reynosa

595 bronze badges

1 Comment

Maximouse Over a year ago

Note that type(a) is str returns False if a is an instance of a str subclass. isinstance(a, str) is better.

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