How to check whether string might be type-cast to float in Python?

As Imran says, your code is absolutely fine as shown.

However, it does encourage clients of isFloat down the "Look Before You Leap" path instead of the more Pythonic "Easier to Ask Forgiveness than Permission" path.

It's more Pythonic for clients to assume they have a string representing a float but be ready to handle the exception that will be thrown if it isn't.

This approach also has the nice side-effect of converting the string to a float once instead of twice.

A more complete generalization:

    def strType(xstr):
        try:
            int(xstr)
            return 'int'
        except:
            try:
                float(xstr)
                return 'float'
            except:
                try:
                    complex(xstr)
                    return 'complex'
                except:
                    return 'str'


    print("4", strType("4"))
    print("12345678901234567890", strType("12345678901234567890"))
    print("4.1", strType("4.1"))
    print("4.1+3j", strType("4.1+3j"))
    print("a", strType("a"))

There's a catch though: float('nan') returns nan, no exception raised ;)

Your code is absolutely fine. Regex based solutions are more likely to be error prone.

Quick testing with timeit reveals float(str_val) is indeed faster than re.match()

>>> timeit.timeit('float("-1.1")')
1.2833082290601467
>>> timeit.timeit(r"pat.match('-1.1')", "import re; pat=re.compile(r'^-?\d*\.?\d+(?:[Ee]-?\d+)?$');")
1.5084138986904527

And the regex used above fails one edge case, it can't match '-1.', although float() will happily convert it to proper float value.