Check if string is float expressed as a decimal number only

Question

I'm writing script for checking if a pair of numbers is a valid coordinate. I need to check if the numbers are expressed as decimals only and in the range of 0 to 180 positive or negative for longitude and 0 to 90 positive or negative for latitude. I have used a try/except block to check if the number is a float like this:

def isFloat(n):
    try:
       float(n)
       return True
    except ValueError:
       return False

While this mostly works, I want it to accept floats expressed only as decimals and not values like True, False, 1e1, NaN

surprisingly this is not a duplicate! +1!!

Jean-François Fabre
– Jean-François Fabre ♦

2017-01-29 16:50:33 +00:00
Commented Jan 29, 2017 at 16:50 — Jean-François Fabre
– Jean-François Fabre ♦, Commented Jan 29, 2017 at 16:50

user2390182 · Accepted Answer · 2017-01-29 17:04:16Z

2

You could use a fairly simple regular expression:

import re

def isFloat(n):
    n = str(n)  # optional; make sure you have string
    return bool(re.match(r'^-?\d+(\.\d+)?$', n))  # bool is not strictly necessary
    # ^         string beginning
    # -?        an optional -
    # \d+       followed by one or more digits (\d* if you want to allow e.g. '.95')
    # (\.\d+)?  followed by an optional group of a dot and one or more digits
    # $         string end

>>> isFloat('4')
True
>>> isFloat('4.567')
True
>>> isFloat('-4.567')
True
>>> isFloat('-4.')
False
>>> isFloat('-4.45v')
False
>>> isFloat('NaN')
False
>>> isFloat('1e1')
False

edited Jan 29, 2017 at 17:04

answered Jan 29, 2017 at 16:47

user2390182

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4 Comments

Tony Tuttle Over a year ago

I would change the first d+ to d* to catch values like .985. Also, while I think this solution is the best for the poster's needs, it does require n to be (cast to) a string.

Tirth Jain Over a year ago

I'm afraid I'm just a novice so could you please explain how this works?

user2390182 Over a year ago

@TirthJain I added some explanation and added the link to the docs of pythons regex module. As for the ins and outs of regular expression syntax, you should read the docs or do a tutorial ;)

user2390182 Over a year ago

@huck_cussler I added that option in the comment, but I don't really like it as it would accept the empty string... (without further complicating the expression) As for the casting, I took it from the question's title that n is a string.

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