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 print statement puts a newline after the output: print("Here is one print statement") print("And here is another") prints Here is one print statement And here is another 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 Printing

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 \pi 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:¹ 3.14 6.28 9.42 12.57

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” takes 5 spaces to print and the rest take 4). So, we use the following format string:

"%5.2f"

When you make this change to the above print, you get: 3.14 6.28 9.42 12.57

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 %(Q)03d quote types.' % {"Q":2, 'language' : "Python"}) gives Python has 002 quote types. The 'language':"Python" associates the string “Python” with the name “language”, so %(language)s means to print a string (the last “s) using the value associated with the name “language”, which is the string “Python”. Similarly, the name “Q” is associated with the integer 2, so %(Q)03d prints the value associated with “Q” (that is, 2) in a field of width 3 with leading 0s (the “03” in front of the “d”).

You can provide a format specification to control how the arguments to format are printed. For example:

import math
print("The value of pi is {0:.7f}".format(math.pi))

You get The value of pi is 3.1415927

You can also control positioning within the field:

print("This prints to the left in the field:   '{0:<6}'".format('text'))
print("This prints to the right in the field:  '{0:>6}'".format('text'))
print("This prints in the center of the field: '{0:^6}'".format('text'))

produces This prints to the left in the field: 'text ' This prints to the right in the field: ' text' This prints in the center of the field: ' text ' Of course, this works with numbers too. It's particularly useful for printing tables!

What Is Actually Going On

Both of these are simply ways to write a string. Ignore the word print.

In the first method, 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:

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

and you will see 2 * 3.141593 = 6.283185

The same thing is true for the format method. Again, to see this, type the following to the IDLE interpreter:

import math
x = "2 * {0:.6f} = {1:.6f}".format(math.pi, 2*math.pi)
print(x)

and you will again 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.

You can also obtain a PDF version of this.

Version of March 29, 2014 at 10:29PM