Python string format: When to use !s conversion flag

Question

What\'s the difference between these 2 string format statements in Python:

\'{0}\'.format(a)
\'{0!s}\'.format(a)

Both have the same output

Answer 1

It is mentioned in the documentation:

The conversion field causes a type coercion before formatting. Normally, the job of formatting a value is done by the __format__() method of the value itself. However, in some cases it is desirable to force a type to be formatted as a string, overriding its own definition of formatting. By converting the value to a string before calling __format__(), the normal formatting logic is bypassed.

Two conversion flags are currently supported: '!s' which calls str() on the value, and '!r' which calls repr().

An example can be taken (again from the documentation) to show the difference:

>>> "repr() shows quotes: {!r}; str() doesn't: {!s}".format('test1', 'test2')
"repr() shows quotes: 'test1'; str() doesn't: test2"