While loop doesn't stop

Question

I have this simple code in Python:

import sys
class Crawler(object):
 def __init__(self, num_of_runs):
 self.run_number = 1
 self.num_of_runs = num_of_runs
 def single_run(self):
 #do stuff
 pass
 def run(self):
 while self.run_number <= self.num_of_runs:
 self.single_run()
 print self.run_number
 self.run_number += 1
if __name__ == "__main__":
 num_of_runs = sys.argv[1]
 crawler = Crawler(num_of_runs)
 crawler.run()

Then, I run it this way:

python path/crawler.py 10

From my understanding, it should loop 10 times and stop, right? Why it doesn't?

• Paco: 10 is a string. You need to convert it to an int with the int() function.
• tom: See this for how Python handles comparisons between ints and strings stackoverflow.com/questions/3270680/…

Answer 1

num_of_runs = sys.argv[1]

num_of_runs is a string at that stage.

while self.run_number <= self.num_of_runs:

You are comparing a string and an int here.

A simple way to fix this is to convert it to an int

num_of_runs = int(sysargv[1])

Another way to deal with this is to use argparser.

import argparse
parser = argparse.ArgumentParser(description='The program does bla and bla')
parser.add_argument(
 'my_int',
 type=int,
 help='an integer for the script'
)
args = parser.parse_args()
print args.my_int
print type(args.my_int)

Now if you execute the script like this:

./my_script.py 20

The output is:

20

Using argparser also gives you the -h option by default:

python my_script.py -h
usage: i.py [-h] my_int
The program does bla and bla
positional arguments:
 my_int an integer for the script
optional arguments:
 -h, --help show this help message and exit

For more information, have a look at the argparser documentation.

Note: The code I have used is from the argparser documentation, but has been slightly modified.

Answer 2

When accepting input from the command line, data is passed as a string. You need to convert this value to an int before you pass it to your Crawler class:

num_of_runs = int(sys.argv[1])

---

You can also utilize this to determine if the input is valid. If it doesn't convert to an int, it will throw an error.