Week 14 agenda and readings -- Python conditionals and files

30 Nov 2011

Review questions

  1. Look at the following function:

    def f(x):
        x = 2 * x
        return x

    What are the values of x and y after running the following lines?

    x = 5
    y = f(x)
  2. Write a function that takes a string and a number n as input, and prints the string n times.

  3. Write a function that doubles every entry in a list of numbers.
    Use it like the following lines:

    numbers = [2,3,4]
    double(numbers)    # numbers is now [4,6,8]

    Why can you modify the variable within the function like this?


To execute a line only if a condition is met, Python provides an if statement. For example, if we have a variable called hour that contains the current hour of the day, we can say "good morning" if necessary:

if hour < 12:
    print('Good morning!')

We can add an else statement to print a message for the other cases:

if hour < 12:
    print('Good morning!')
    print('Good not-morning!')

If we have a whole sequence of decisions to make, we can fill in the middle with elif statements:

if hour > 5 and hour < 12:
    print('Good morning!')
elif hour < 18:
    print('Good afternoon!')
elif hour < 23:
    print('Good evening!')
    print('Good night!')

Some common uses for conditionals include:

  • Keeping track of the largest or smallest element in a for loop. Try to define functions to find the min or max number in a list.
  • Defining piecewise functions. Try to define the absolute value function using an if-else statement.


The open command opens a file for reading or writing. It returns an object that represents the file; you can call its methods to get the file contents (or write to the file if you've opened it for writing).

Here are some examples, assuming you've got Huck Finn saved in your working directory.

huck = open('pg76.txt')
contents = huck.read()
print('The file contains', len(contents), 'characters')

output = open('output.txt', 'w')  # creates or overwrites file output.txt
print('This message goes in the file', file=output)

Files are automatically closed when your program ends, but it's good practice to use some method to make sure they're closed as soon as possible (so that other programs can use them).

One of the most useful ways to deal with files is to iterate through them line-by-line using a for-loop. File objects work just like lists in this context (they provide the same iterable interface).

There are two special file objects available in all programs through the sys module. stdin is the file object that provides input from the keyboard (or from a file if input redirection is used on the command line). stdout is the file object that is used to write output (this is where output from the print function goes by default).

Command line arguments

When we used command line programs earlier in the semester, we typed a command followed by a number of arguments. We can do the same with our Python programs. The sys module has a special variable called argv that contains those values. Try running the following program:

import sys

argv is just a list of the strings that you typed on the command line.


Using Huck Finn again, let's re-create some of the programs we used before:

  1. Create a program that counts the number of lines in the file specified on the command line. So if you type: linecount FILENAME, it will print the number of lines in FILENAME. Create an alternate version that counts the lines from stdin if no FILENAME is specified.

  2. Create a program that counts the number of lines containing a particular word. So if you type wordlines WORD FILENAME, it will print the number of lines that contain WORD in the file FILENAME. Create an alternate version that prints all those lines instead of counting them. Try using this version in a pipeline with your linecount program.

  3. Create a program that finds the longest word in a file. Hints:

    • words = line.split() will take a line and split it into a list of words.
    • Break the program up into small functions. Define a function that finds the longest word in a list of words. Then define a function that finds the longest word in the entire file, using the first function.
    • For finding the longest word, think about how you defined the max function to find the max number in a list. You can find the longest word in almost the same way -- you just need to transform the input in some way.


Read chapters 5 and 6 in How to Think Like a Computer Scientist: Learning with Python. As usual, do as many exercises as you can.