I have a function that takes an optional argument like so:
myProgram -n 8
I want to add in error handling that will exit the program and print an error message if the argument that the user enters is a float. How would I test for this if the argument always comes in as a string?
It may seem heavy-weight at first, but the argparse module can do exactly what you want. The first example on the page shows an integer-only argument (notice the type=int part):
parser = argparse.ArgumentParser(description='Process some integers.')
parser.add_argument('integers', metavar='N', type=int, nargs='+',
help='an integer for the accumulator')
Calling that example with python deleteme.py 3.5 (i.e. 3.5 is the argument that should be an integer) gives the following output:
usage: scriptname.py [-h] [--sum] N [N ...]
scriptname.py: error: argument N: invalid int value: '3.5'
Use str.isdigit():
>>> '12345'.isdigit()
True
>>> '12.345'.isdigit()
False
If you want to support negative numbers, strip the sign off first:
>>> '+12345'.strip(' -+').isdigit()
True
>>> '-12345'.strip(' -+').isdigit()
True
To parse command line arguments, you should be using argparse.
import argparse
def main():
parser = argparse.ArgumentParser()
parser.add_argument("-n", type=int)
args = parser.parse_args()
print args
if __name__ == '__main__':
main()
Usage:
[10:39pm][wlynch@watermelon /tmp] python blah.py
Namespace(n=None)
[10:39pm][wlynch@watermelon /tmp] python blah.py -n 4
Namespace(n=4)
[10:39pm][wlynch@watermelon /tmp] python blah.py -n 4.0
usage: blah.py [-h] [-n N]
blah.py: error: argument -n: invalid int value: '4.0'
You could perhaps do something like this:
from math import modf
if modf(float(args.n))[0] != 0.0:
# ERROR: Float entered!
From the math.modf docs:
math.modf(x)¶ Return the fractional and integer parts of x. Both results carry the sign of x and are floats.
Or alternatively (as @dawg mentioned):
"." in args.n # A possible float
# or
args.n.isdigit() # Returns False if args.n does not contain all digits only
