Allow range function to evaluate non numerical expression

I am using a makearange function to input a start, stop, and increment range for an array i.e.

User = raw_input('Enter start,[stop],[increment]: ').split(',')
makearange = lambda a: numpy.arange(int(a[0]),int(a[1]),int(a[2]))
x = makearange(User)

However I am also using these numbers to run a program to create arrays of the squares and cubes of the inputted numbers. I am running this program on an infinite while loop, which only stops when the user hits the return key. So I have tried

if User == "":
    Break

Which would work except this results in an error because the m开发者_开发百科akearange function only evaluates integers and not the user input of the return key. How can I get it to understand this type of input? Thanks

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Instead of immediately trying to split User on commas, test if it is the empty string first:

import numpy as np
import sys

user_input = raw_input('Enter [start,] stop[, increment]: ')
if user_input = '':
    sys.exit()
else:
    x=np.arange(*map(int,user_input.split(',')))

PS. Enter start,[stop],[increment] suggests that stop and increment are optional. Does that mean if only one argument is given, you want the range to start from the given number and increase infinitely? That's won't work with numpy.arange. Perhaps you meant the start to be optional, and the stop to be required. That would fit perfectly with the way numpy.arange already works.

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makearange = lambda a: numpy.arange(int(a[0]),int(a[1]),int(a[2])) if len(a) == 3 else None

of course you can come up with a better validation than the length of the array