Python format()

The built-in format() method returns a formatted representation of the given value controlled by the format specifier.

The format() function is similar to the String format method. Internally, both methods call __format__() method of an object.

While the built-in format() function is a low level implementation for formatting an object using __format__() internally, string format() is a higher level implementation able to perform complex formatting operations on multiple object strings as well.

The syntax of format() is:

format(value[, format_spec])

1. format() Parameters

The format() function takes two parameters:

• value – value that needs to be formatted
• format_spec – The specification on how the value should be formatted.

The format specifier could be in the format:

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

Visit these links to learn more about format types and alignment.

2. Return value from format()

The format() function returns a formatted representation of a given value specified by the format specifier.

3. Example 1: Number formatting with format()

# d, f and b are type

# integer
print(format(123, "d"))

# float arguments
print(format(123.4567898, "f"))

# binary format
print(format(12, "b"))

Output

123
123.456790
1100

4. Example 2: Number formatting with fill, align, sign, width, precision and type

# integer 
print(format(1234, "*>+7,d"))

# float number
print(format(123.4567, "^-09.3f"))

Output

*+1,234
0123.4570

Here, when formatting the integer 1234, we’ve specified the formatting specifier *>+7,d. Let’s understand each option:

• * – It is the fill character that fills up the empty spaces after formatting
• > – It is the right alignment option that aligns the output string to the right
• + – It is the sign option that forces the number to be signed (having a sign on its left)
• 7 – It is the width option that forces the number to take a minimum width of 7, other spaces will be filled by fill character
• , – It is the thousands operator that places a comma between all thousands.
• d – It is the type option that specifies the number is an integer.

When formatting the floating point number 123.4567, we’ve specified the format specifier ^-09.3f. These are:

• ^ – It is the center alignment option that aligns the output string to the center of the remaining space
• - – It is the sign option that forces only negative numbers to show the sign
• 0 – It is the character that is placed in place of the empty spaces.
• 9 – It is the width option that sets the minimum width of the number to 9 (including decimal point, thousands comma and sign)
• .3 – It is the precision operator that sets the precision of the given floating number to 3 places
• f – It is the type option that specifies the number is a float.

5. Example 3: Using format() by overriding __format__()

# custom __format__() method
class Person:
    def __format__(self, format):
        if(format == 'age'):
            return '23'
        return 'None'

print(format(Person(), "age"))

Output

23

Here, we have overridden the __format__() method of the class Person.

It now accepts a format parameter and returns 23 if it is equal to 'age'. If no format is specified, None is returned.

The format() function internally runs Person().__format__("age") to return 23.