Tips & Tutorials

The main use I’ve found for the Python sys module is allowing command-line arguments to be made to a script. Here is an example of how it looks:

import sys

if len(sys.argv) == 2:
    input_file = sys.argv[1]
else:
    print "Please input a command-line argument specifying the file"

This script checks that 2 command-line arguments had been passed to the program before assigning the value sys.argv[1] to a variable. We check for two command-line arguments because the first one (sys.argv[0]) is the name of the Python script currently being executed.

You can use the Python os module to send commands out to the operating system. Through os you can do anything from changing the current working directory through to listing the contents of a directory.

import os

# print the current working directory
print os.getcwd()
# change the working directory
os.chdir("..")
# create a new directory
os.mkdir("new_folder")
# list the contents of a directory
os.listdir("new_folder")
# remove directory
os.rmdir("new_folder")
# remove a file
os.remove("image.jpg")
# rename a file/directory and where it will go
os.rename("/data/jack", "/james")

Almost anyone doing research at some point will have to input or extract data from an Excel spreadsheet. However, it is possible to export an Excel file as a .csv file (comma separated file) and then use Python to grab all the data instantly. The Python csv module makes all this incredibly simple:

import csv
column1 = []
column2 = []
column3 = []
openfile = csv.reader(open('data.csv'))
for row in openfile:
    first, second, third = row
    column1.append(first)
    column2.append(second)
    column3.append(third)

This script will open your .csv file, and then use the ‘.next()’ method to create an array of all the lines in your file, which you can parse through with the for loop.