如何检查字符串是否为十进制/浮点数？

Question

I need to check if a string is of the form of a decimal/ float number.

I have tried using isdigit() and isdecimal() and isnumeric(), but they don't work for float numbers. I also can not use try: and convert to a float, because that will convert things like " 12.32" into floats, even though there is a leading white-space. If there is leading white-spaces, I need to be able to detect it and that means it is not a decimal.

I expect that "5.1211" returns true as a decimal, as well as "51231". However, something like "123.12312.2" should not return true, as well any input with white-space in it like " 123.12" or "123. 12 ".

Answer 1

By no means am I suggesting this is the best way of doing things as I am a beginner myself, but, you could do something like:

try:
    if num[0] != " " and num[-1] != " ":
        num = float(num)
        is_float = True
except ValueError:
    is_float = False

This is very similar to @jthecoder 's answer however it also accounts for the white space.

EDIT: @jthecoder I didn't see the author mention strings ending with white spaces because it cuts off midline for me. My code now meets every requirement of the author.

Answer 2

This is a good use case for regular expressions .

You can quickly test your regex patterns at https://pythex.org/ .

import re

def isfloat(item):

    # A float is a float
    if isinstance(item, float):
        return True

    # Ints are okay
    if isinstance(item, int):
        return True

   # Detect leading white-spaces
    if len(item) != len(item.strip()):
        return False

    # Some strings can represent floats or ints ( i.e. a decimal )
    if isinstance(item, str):
        # regex matching
        int_pattern = re.compile("^[0-9]*$")
        float_pattern = re.compile("^[0-9]*.[0-9]*$")
        if float_pattern.match(item) or int_pattern.match(item):
            return True
        else:
            return False

assert isfloat("5.1211") is True
assert isfloat("51231") is True
assert isfloat("123.12312.2") is False
assert isfloat(" 123.12") is False
assert isfloat("123.12 ") is False
print("isfloat() passed all tests.")

Answer 3

Use a try catch loop, and see if it throws an error if you convert it. Sample code:

try: 
  num_if_float = float(your_str_here)
  str = true
except:
  float = false