11. Formatting Strings - Old Style

So far, we’ve been using string concatenation and multiple arguments to print() to format strings. In this notebook, we will use printf style formatting to put values into strings with a variety of different formats.

Python offers three different ways to format strings:

• old style (all versions of Pythons) - this notebook

• new style (Python 2.6 +)

• f-strings (Python 3.6 +)

In this notebook, we are directly printing the results of the formatting, but we can also assign the results to a variable or use as an expression (e.g., as an argument to a function).

11.1. Old style: %

This format uses the form of format_string % data. Within each string are one of more placeholder fields that will be replaced by values after the % operator.

Note: The book Introducing Python does considers this formatting to be string interpolation as the format strings contain placeholders. Others strictly require expressions to be supported to be considered true string interpoloation (which exists in f-strings).

While development and support of Python 2 has ended, the old style formatting will remain in Python for the forseeable future.

Placeholders follow this format: %[flags][minWidth][.maxchars]type

The square brackets indicate an optional component.

Type	Description
%s	string
%d	integer
%x	hexadecimal integer
%o	octal integer
%f	float
%e	float in exponential format
%g	decimal or exponential float
%%	the character %

For the optional components -

The flags have the possible options

• - left aligns the output of the placeholder.

• + Prepends a plus for positive signed-numeric types

• 0 will prepend zeros for numeric types if the width is present

The minwidth is the minimum size of the placeholder.

If the type is a string, .maxchars specifies how many characters to print from the data value.
If the type is a float, .maxchars specifies the precision - the number of digits after the decimal point. If the type is an integer, .maxchars specifies the length of the integer to print. zeros will be prepended if the number is less than that length.

alphabet = "abcdefghijklmnopqrstuvwxyz"
print("123456789012345678901234567890")
print("%s" % alphabet)
print("%.5s" % alphabet)
print("%30.5s" % alphabet)
print("%-30.5s" % alphabet)

123456789012345678901234567890
abcdefghijklmnopqrstuvwxyz
abcde
                         abcde
abcde                         

year = 1892
print("123456789012345678901234567890")
print("%d" % year)
print("%20d" % year)
print("%+20d" % year)
print("%020d" % year)
print("%20.8d" % year)
print("%-20d" % year)
print("%x" % year)
print("%o" % year)

123456789012345678901234567890
1892
                1892
               +1892
00000000000000001892
            00001892
1892                
764
3544

gain = 64.98712
print("123456789012345678901234567890")
print("%f" % gain)
print("%20f" % gain)
print("%+20f" % gain)
print("%020f" % gain)
print("%20.8f" % gain)
print("%20.2f" % gain)
print("%-20f" % gain)

123456789012345678901234567890
64.987120
           64.987120
          +64.987120
0000000000064.987120
         64.98712000
               64.99
64.987120           

gain = 604132421.988
print("123456789012345678901234567890")
print("%e" % gain)
print("%20e" % gain)
print("%+20e" % gain)
print("%020e" % gain)
print("%20.8e" % gain)
print("%20.2e" % gain)
print("%-20e" % gain)
print("%20.12e" % gain)

123456789012345678901234567890
6.041324e+08
        6.041324e+08
       +6.041324e+08
000000006.041324e+08
      6.04132422e+08
            6.04e+08
6.041324e+08        
  6.041324219880e+08

Of course, we can include other content within these strings

print("In %d, Duke University moved to Durham, North Carolina." % 1892)

In 1892, Duke University moved to Durham, North Carolina.

You can also specify multiple placeholders within a string. The values will put inside of a tuple (which you will see in Notebook 3d). For right now, you just need to know that tuples are bound by paranthesis with values separated inside by commas

year = 1838
school = "Duke University"
city = "Trinity, North Carolina"
print("In %d, %s was founded in %s." % (year,school,city))
year = 1842
school = "the University of Notre Dame"
city = "South Bend, Indiana"
print("In %d, %s was founded in %s." % (year,school,city))

In 1838, Duke University was founded in Trinity, North Carolina.
In 1842, the University of Notre Dame was founded in South Bend, Indiana.