Formatting Output

Formatting With Print (Review)

How can we format output with print (that is, control what gets printed out between separate arguments, and what gets printed out after the actual value)? →

Let's check out some examples in the next slide. →

Formatting With Print Examples

• note that there are no quotes around sep and end
• these keyword arguments go after regular arguments

// instead of using space to separate arguments, use comma!
print("foo", "bar", "baz", sep=", ")

// use a + instead of a newline at the end!
print("foo", "bar", "baz", end="+")

// you can combine both sep and end
print("foo", "bar", "baz", sep=", ", end="+")

The format() Function

The format() function can be used to format a value before you use it elsewhere (for example, before you print out to the user)

• it takes two arguments
  • the thing (value) you want formatted (can be a string or numeric type)
  • some formatting pattern (expressed as a string)
• format() always returns a string
• the original value is not modified in any way!

Padding Strings

You can use format to ensure that you string has some number of characters in it.

• to fill out the remaining characters, format will add spaces before or after your original string
• this is specified by the format string
• for example, '>20s' and '<20s' add spaces to the beginning and end of a string…

formatted = format('hello', '>20s')
print(formatted)

Examples of Formatting Floats

Specifying places after decimal (. means decimal, number is the number of spaces, f means floating point format)

a = 1.333333
b = format (a, '.2f') # format as a float, 1.33
c = format (a, '.3f') # format as a float, 1.333
d = format (a, ',.5f') # add a comma separator
e = format (a, '>20,.2f') # pad it too!

Examples of Formatting Integers

Specifying places after decimal (, means add comma, d means integer)

a = 20000
print(format(a, ',d'))
print(format(a, '>20,d'))

The Formal Specification

Ok… so if you want to know how format really works:

• the second argument is a string that represents a format
• and it's written in a "mini-language" (yes, a language in a language!)

format_spec ::=  [[fill]align][sign][#][0][width][,][.precision][type]
fill        ::=  <any character>
align       ::=  "<" | ">" | "=" | "^"
sign        ::=  "+" | "-" | " "
width       ::=  integer
precision   ::=  integer
type        ::=  "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | 
                 "G" | "n" | "o" | "s" | "x" | "X" | "%"

(taken from the official documentation)

Additional Notes

There are a few other minor details with format →

• you can leave out the type of you're not doing any specific formatting based on that type
  • format(1.2)
• if you use a type with an incompatible value, you'll get an error
  • format(1.2, 's')
• notice the other possible items that you can add to the format specifier?
  • you can try adding things like fill, sign, etc.

Let's Try This…

What format string should we use to format the number 1000.32 so that it gives back a stirng composed of 20 characters: →