Python String Formatting

String Formatting

The format() method is used to perform a string formatting operation. The string on which this method is called can contain literal text or replacement fields delimited by braces {}. Each replacement field contains either the numeric index of a positional argument or the name of a keyword argument.

Syntax:

str.format(*args, **kwargs)

Returns a copy of the string where each replacement field is replaced with the string value of the corresponding argument.

Basic formatting:

Example-1:

>>> '{} {}'.format('Python', 'Format')
'Python Format'
>>> 

>>> '{} {}'.format(10, 30)
'10 30'
>>>

This following statement allows re-arrange the order of display without changing the arguments.

Example-2:

>>> '{1} {0}'.format('Python', 'Format')
'Format Python'
>>>

Value conversion:

The new-style simple formatter calls by default the __format__() method of an object for its representation. If you just want to render the output of str(...) or repr(...) you can use the !s or !r conversion flags.

In %-style you usually use %s for the string representation but there is %r for a repr(...) conversion.

Setup:

class Data(object):

    def __str__(self):
        return 'str'

    def __repr__(self):
        return 'repr'

Example-1:

class Data(object):

    def __str__(self):
        return 'str'

    def __repr__(self):
        return 'repr'
x='{0!s} {0!r}'.format(Data())
print (x)

Output:

str repr

In Python 3 there exists an additional conversion flag that uses the output of repr(...) but uses ascii(...) instead.

Example-2:

class Data(object):

    def __repr__(self):
        return 'räpr'
x='{0!r} {0!a}'.format(Data())
print(x)

Output:

räpr r\xe4pr

Padding and aligning strings:

A value can be padded to a specific length. See the following examples where the value '15' is encoded as part of the format string.

Note: The padding character can be spaces or a specified character.

Example:

Align right:

>>> '{:>15}'.format('Python')
'         Python'
>>>

Align left:

>>> '{:15}'.format('Python')
'Python         '
>>>

By argument:

In the previous example, the value '15' is encoded as part of the format string. It is also possible to supply such values as an argument.

Example:

>>> '{:<{}s}'.format('Python', 15)
'Python         '
>>>

In the following example we have used '*' as a padding character.

Example:

>>> '{:*<15}'.format('Python')
'Python*********'
>>>

Align center:

Example:

>>> '{:^16}'.format('Python')
'     Python     '
>>>

Truncating long strings:

In the following example, we have truncated ten characters from the left side of a specified string.

Example:

>>> '{:.10}'.format('Python Tutorial')
'Python Tut'
>>>

By argument:

Example:

>>> '{:.{}}'.format('Python Tutorial', 10)
'Python Tut'
>>>

Combining truncating and padding

In the following example, we have combined truncating and padding.

Example:

>>> '{:10.10}'.format('Python')
'Python    '
>>>

Numbers:

Integers:

>>> '{:d}'.format(24)
'24'
>>> 

Floats:

>>> '{:f}'.format(5.12345678123)
'5.123457'
>>>

Padding numbers:

Similar to strings numbers.

Example-1:

>>> '{:5d}'.format(24)
'   24'
>>>

The padding value represents the length of the complete output for floating points. In the following example '{:05.2f}' will display the float using five characters with two digits after the decimal point.

Example-2:

>>> '{:05.2f}'.format(5.12345678123)
'05.12'
>>>

Signed numbers:

By default only negative numbers are prefixed with a sign, but you can display numbers prefixed with the positive sign also.

Example-1:

>>> '{:+d}'.format(24)
'+24'
>>>

You can use a space character to indicate that negative numbers (should be prefixed with a minus symbol) and a leading space should be used for positive numbers.

Example-2:

>>> '{: d}'.format((- 24))
'-24'
>>>

Example-3:

>>> '{: d}'.format(24)
' 24'
>>>

You can control the position of the sign symbol relative to the padding.

Example-4:

>>> '{:=6d}'.format((- 24))
'-   24'
>>>

Named placeholders:

Both formatting styles support named placeholders. Here is an example:

Example-1:

>>> data = {'first': 'Place', 'last': 'Holder!'}
>>> '{first} {last}'.format(**data)
'Place Holder!'
>>> 

.format() method can accept keyword arguments.

Example-2:

>>> '{first} {last}'.format(first='Place', last='Holder!')
'Place Holder!'
>>>

Datetime:

You can format and print datetime object as per your requirement.

Example:

>>> from datetime import datetime
>>> '{:%Y-%m-%d %H:%M}'.format(datetime(2016, 7, 26, 3, 57))
'2016-07-26 03:57'
>>>

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