% vs format

format

New in version 2.6.

See Format String Syntax for a description of the various formatting options that can be specified in format strings.

This method of string formatting is the new standard in Python 3, and should be preferred to the % formatting described inString Formatting Operations in new code.

In [1]: '{}{}'.format(1, 2)
Out[1]: '12'

In [2]: '{0}{1}{0}'.format(1, 2)
Out[2]: '121'

In [3]: '{one}{two}'.format(one=1, two=2)
Out[3]: '12'

In [2]: '{one}{two}{one}'.format(one=1, two=2)
Out[2]: '121'

In [4]: '{{_}}{one}{two}'.format(one=1, two=2)
Out[4]: '{_}12'

In [5]: '{_}{one}{two}'.format(one=1, two=2)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-5-85a872d71e8b> in <module>()
----> 1 '{_}{one}{two}'.format(one=1, two=2)
KeyError: '_'

• '{} {}' vs. '{0} {1}'
  • Str.format() for Python 2.6 gives error where 2.7 does not

    Changed in version 2.7:
    The positional argument specifiers can be omitted, so '{} {}' is equivalent to '{0} {1}'.
    

%s

In [7]: '%s%s' % (1, 2)
Out[7]: '12'

UnicodeEncodeError

In [1]: '{}{}'.format('test', u'测试')
---------------------------------------------------------------------------
UnicodeEncodeError                        Traceback (most recent call last)
<ipython-input-1-507ab7ed13b6> in <module>()
----> 1 '{}{}'.format('test', u'测试')
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-1: ordinal not in range(128)

In [2]: u'{}{}'.format('test', u'测试')
Out[2]: u'test\u6d4b\u8bd5'

In [3]: '%s%s' % ('test', u'测试')
Out[3]: u'test\u6d4b\u8bd5'

In [4]: from __future__ import unicode_literals

In [5]: '{}{}'.format('test', u'测试')
Out[5]: u'test\u6d4b\u8bd5'

• Python: Using .format() on a Unicode-escaped string

References

[1] PyFormat@TT4IT, PyFormat —— Using % and .format() for great good!

[2] Docs@Python, 6.1. string — Common string operations —— 6.1.3. Format String Syntax