Printing

Python offers several ways to print information. This handout presents two, using the print statement.

Unformatted Printing

Unformatted printing prints numbers and strings in a natural format: integers as ordinary numbers, floating point numbers with two decimal digits, and strings as they are typed. Thus:

num = 1
flnum = 2.6
print("The integer is", num, "and the float is", flnum)

prints

The integer is 1 and the float is 2.6

A comma between two arguments prints a space. The keyword argument end=' ' puts a blank (or whatever string follows the = sign) at the end of the line and suppresses the skip to the next line:

print("Here is one print statement", end=’ ')
print("And here is another")

prints

Here is one print statement And here is another

One problem with unformatted printing is that you cannot avoid adding spaces before numbers. So, if you try to print “$1.39” with this:

print("$", 1.39)

you get

$ 1.39

(notice the unwanted space). To print this, you need a formatted print statement.

Formatted Print

A formatted print statement allows you to control how the number or string is printed. You can do lots of things with it that you cannot do with an unformatted print statement. Let’s start with the last unformatted print problem. We want to print “$1.39”. Here's how we do it:

print("$%f" % (1.39))

This prints

$1.390000

The string immediately following the printf is the format string. It is printed as typed, except when a “%” is seen. The sequence of characters following it control how the next arguments are to be printed. “%f” means to print a floating point number. “%d” would mean to print an integer as a decimal number (“d” stands for “decimal”). The “%” after the string says that a tuple containing the arguments for the printing follow.¹ One problem with the above—too many decimal places. Let’s restrict it to 2 decimal places by saying instead

print("$%.2f" % (1.39))

This prints

$1.39

The “%f” says to print the number as a floating point number. The “.2” between the “%” and “f” means to print 2 digits after the decimal point. As another example, if you want to print the value of π to 7 decimal places, say:

import math
print("The value of pi is %.7f" % (math.pi))

You get

The value of pi is 3.1415927

Now suppose you want to print a table of numbers. You might say:

for i in range(1, 5):
	print("%.2f" % (math.pi * i))

You get this:²

That doesn’t line up too well! We really want the numbers to the left of the decimal point to take up the same amount of room so the decimal points line up. So, we want to line the numbers up to the right. To do this, we first figure out how many characters (including the decimal point) we want the numbers to take up. That's 5 (because 12.57 has 5 characters and the rest have 4). So, we use the following format string:

"%5.2f"

When you make this change to the above print, you get:

Now let’s say we want to print several arguments. You do it this way:

print "%d + %d = %d" % (2, 2, 2 + 2)

gives

2 + 2 = 4

You can get very fancy:

print(’%(language)s has %(#)03d quote types.' % {"#":2, 'language':"Python"})

gives

Python has 002 quote types.

What Is Actually Going On

Actually, the arguments to print are simply a way to write a string. Ignore the word “print”. Then the string in quotes is called the format string and the parenthesized list following the “%”� are the values. The values are substituted into the format string, resulting in a new string. To see this, type the following to the IDLE interpreter:

x = "2 * %f = %f" % (math.pi, 2*math.pi)
print x

and you will see

2 * 3.141593 = 6.283185

Footnotes

Actually, if there is only one argument to be printed, and that argument is a number, you can omit the parentheses. But if it’s an expression, leaving the parentheses out could cause unexpected results, so it's better to put them in.
Here’s a good example of where the argument requires the parentheses even though only one number is being printed. Try leaving them out and see what happens.

A PDF version is available here.

ECS 10, Basic Concepts of Computing
Fall Quarter 2012