Printing

Python offers several ways to print information. This handout presents three, using the print statement. First is unformatted printing, in which what you are printing is printed as is. Then comes formatted printing, which allows you to control how what you are printing looks. Finally, the format method provides the same capability. Python 3 programmers tend to prefer the latter when doing formatted printing, but either works in Python 3.

Unformatted Printing

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

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

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")

Here is one print statement
And here is another

Ending the print statement with a comma at the end of the line puts in a blank and suppresses the skip to the next line:

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

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)

$ 1.39

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))

$1.390000

The string immediately following the print 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))

$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))

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))

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"

 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))

2 + 2 = 4

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

Python has 002 quote types.

Format Method

Python offers a second way to print. It uses a method called format.

Here’s the basic form:

print( "{} is different than {}".format("Python 2", "Python 3"))

Python 2 is different than Python 3

If you want to refer to a specific argument of format, you can. Just put the number of the argument in the braces. So, in

print( "{0} {1} is different than {0} {2}".format("Python", 2, "3"))

Names work, too:

print(’{language}s have {Q:03d} quote types.’.format(Q=2, language="Python"))

Pythons have 002 quote types.

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))

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’))

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 ’

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)

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)

2 * 3.141593 = 6.283185

Table of print Format Characters

Here is the list of the formatting characters and what they do.

Footnotes

1. 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.
2. 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.

Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu

MHI 289I, Programming in Health Informatics Version of September 20, 2021 at 4:13PM

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