Using isdigit for floats?

Question

a = raw_input(\'How much is 1 share in that company? \')

while not a.isdigit():
    print(\"You need to write a number!\\n\")
    a = raw_input(\'How much is 1 shar         


        

Answer 1

The provided answers fail if the string contains some special characters such as underscore (e.g. '1_1'). The following function returns correct answer in all case that I tested.

def IfStringRepresentsFloat(s):
try:
    float(s)
    return str(float(s)) == s
except ValueError:
    return False

Answer 2

import re

string1 = "0.5"
string2 = "0.5a"
string3 = "a0.5"
string4 = "a0.5a"

p = re.compile(r'\d+(\.\d+)?$')

if p.match(string1):
    print(string1 + " float or int")
else:
    print(string1 + " not float or int")

if p.match(string2):
    print(string2 + " float or int")
else:
    print(string2 + " not float or int")

if p.match(string3):
    print(string3 + " float or int")
else:
    print(string3 + " not float or int")

if p.match(string4):
    print(string4 + " float or int")
else:
    print(string4 + " not float or int")

output:
0.5 float or int
0.5a not float or int
a0.5 not float or int
a0.5a not float or int

Answer 3

EAFP

try:
    x = float(a)
except ValueError:
    print("You must enter a number")

Answer 4

s = '12.32'
if s.replace('.', '').replace('-', '').isdigit():
    print(float(s))

Note that this will work for negative floats as well.

Answer 5

I think @dan04 has the right approach (EAFP), but unfortunately the real world is often a special case and some additional code is really required to manage things—so below is a more elaborate, but also a bit more pragmatic (and realistic):

import sys

while True:
    try:
        a = raw_input('How much is 1 share in that company? ')
        x = float(a)
        # validity check(s)
        if x < 0: raise ValueError('share price must be positive')
    except ValueError, e:
        print("ValueError: '{}'".format(e))
        print("Please try entering it again...")
    except KeyboardInterrupt:
        sys.exit("\n<terminated by user>")
    except:
        exc_value = sys.exc_info()[1]
        exc_class = exc_value.__class__.__name__
        print("{} exception: '{}'".format(exc_class, exc_value))
        sys.exit("<fatal error encountered>")
    else:
        break  # no exceptions occurred, terminate loop

print("Share price entered: {}".format(x))

Sample usage:

> python numeric_input.py
How much is 1 share in that company? abc
ValueError: 'could not convert string to float: abc'
Please try entering it again...
How much is 1 share in that company? -1
ValueError: 'share price must be positive'
Please try entering it again...
How much is 1 share in that company? 9
Share price entered: 9.0

> python numeric_input.py
How much is 1 share in that company? 9.2
Share price entered: 9.2

Answer 6

Use regular expressions.

import re

p = re.compile('\d+(\.\d+)?')

a = raw_input('How much is 1 share in that company? ')

while p.match(a) == None:
    print "You need to write a number!\n"
    a = raw_input('How much is 1 share in that company? ')