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))

which 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 ofπ 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 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

ECS 10, Basic Concepts of Computing Winter Quarter 2012