Testing python programs without using python shell

Question

I would like to easily test my python programs without constantly using the python shell since each time the program is modified you have to quit, re-enter the python shell and import the program again. I am using a 2012 Macbook pro with OSX. I have the following code:

import sys

def read_strings(filename):
    with open(filename) as file:
        return file.read().split('>')[1:0]

file1 = sys.argv[1]
filename = read_strings(file1)

Essentially I would like to read into and split a txt file containing:

id1>id2>id3>id4

I am entering this into my command line:

pal-nat184-102-127:python_stuff ceb$ python3 program.py string.txt

However when I try the sys.argv approach on the command line my program returns nothing. Is this a good approach to testing code, could anyone point me in the correct direction?

This is what I would like to happen:

pal-nat184-102-127:python_stuff ceb$ python3 program.py string.txt

['id1', 'id2', 'id3', 'id4']

1. Quit from where and re-import where? 2. Yes, running script/test suite from the shell is a common method of testing. 3. Of course it does not return anything, because you do not tell it to return anything — what is the result you expect? — fjarri
– fjarri, Commented Oct 10, 2013 at 0:35

Behram Mistree · Accepted Answer · 2013-10-10 00:40:22Z

Let's take this a piece at a time:

However when I try the sys.argv approach on the command line my program returns nothing

The final result of your program is that it writes a string into the variable filename. It's a little strange to have a program "return" a value. Generally, you want a program to print it's something out or save something to a file. I'm guessing it would ease your debugging if you modified your program by adding,

print (filename)

at the end: you'd be able to see the result of your program.

could anyone point me in the correct direction?

One other debugging note: It can be useful to write your .py files so that they can be run both independently at the command line or in a python shell. How you've currently structured your code, this will work semi-poorly. (Starting a shell and then importing your file will cause an error because sys.argv[1] isn't defined.)

A solution to this is to change your the bottom section of your code as follows:

if __name__ == '__main__':
    file1 = sys.argv[1]
    filename = read_strings(file1)

The if guard at the top says, "If running as a standalone script, then run what's below me. If you imported me from some place else, then do not execute what's below me."

Feel free to follow up below if I misinterpreted your question.

Joseph · Accepted Answer · 2013-10-10 00:49:19Z

1

You never do anything with the result of read_strings. Try:

print(read_strings(file1))

edited Oct 10, 2013 at 0:49

answered Oct 10, 2013 at 0:30

Joseph

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4 Comments

Behram Mistree Over a year ago

Minor note: in Python 3, print is no longer a statement, it's a function. You need to use parens: print(read_strings(file1)) instead of print read_strings(file1).

Joseph Over a year ago

Thanks, I've yet to even touch Python 3 so I appreciate the heads up!

cebach561 Over a year ago

I see. When I use print(read_strings(file1)) the output is the following: []...however I am hoping to get this output: ['id1', 'id2', 'id3', 'id4'].

Joseph Over a year ago

@cebach561 I think the slice you're doing after the call to .split() is the problem. Remove the [1:0] at the end. Does that fix it for you?

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