Programming in the large

1) Functions (Subroutines)

#!/usr/bin/env python

######### modules and global variables ############################
import re
closing_tag = re.compile(r"</.*?>")

######### functions ###############################################
def find_closing_tags(line):
    result = closing_tag.search(line)
    if result:
       print result.group()

######### main program ############################################
file = open("example.html","r")
text = file.readlines()
file.close()

for line in text:
    find_closing_tags(line)

Exercise

1.1 Add a second function to the example that prints all tags that are not closing tags. Before the functions are called, let the main program print a heading "these are the opening tags" and "these are the closing tags". (Note: this script only finds the first tag in every line. To change that so that it finds all tags you would have to insert line breaks after the tags as shown in exercise 4.2 from week 8.)

2) Passing arguments to and from a function

#!/usr/bin/env python

import os

######### functions #############################################
def read_file(filename):
    file = open(filename, "r")
    text = file.readlines()
    file.close()
    return text

def write_file(filename,text):
    file = open(filename, "w")
    file.writelines(text)
    file.close()

######### main program ############################################
filecontent = read_file("example.html")
filecontent.append("this is the last line\n")
write_file("example.html", filecontent)

os.system("more example.html")

The main program sends the file name as argument to the function read_file and receives filecontent from it as return value.

The arguments in the function call and function declaration must appear in the same order.

Exercises

2.1 Write a third function for the script. The function is called get_input. It takes filecontent as argument and also returns it. The function asks a user to input some text. The text is inserted at the end of the file. Optional: the user is also asked for the line where the text will be inserted.

2.2 Modify the previous example so that the user is asked for his/her first name, last name, email address. The information is stored in the file in the following format:
Mary | Smith | mary@somecompany.com
John | Miller | miller@somecompany.com

2.3 A fourth function is added that asks a user for her/his last name and retrieves the email address from the file. To do that every line of the file is split into a list. The second element in the list is compared to the current user's last name.

Optional material: Keyword arguments in the function call

If the argument names from the function declarations are used in the function call, the arguments can be provided in any order. For example, a function
def write_file(filename,text):
can be called with
write_file(text =filecontent, filename= "example.html")

Optional material: Default arguments in the function declaration

Default arguments can be provided in a function declaration. But these must be listed after non-default arguments. For example, the write_file function can be declared as
def write_file(text,filename = "defaultfile.html"):
 and be called with
write_file(text =filecontent)

Optional material: Local and Global Variables

Each function should use its own, local set of variables so that the different functions do not interfere with each other. "Global variables" are variables that are used in the main part. These are available in all functions. In the script under 1), a global variable was used: closing_tag. The use of global variables should be avoided.