Understanding Python’s !s and !r Format Specifiers and the Difference Between __str__ and __repr__

This article introduces the use of the !s and !r conversion flags in Python’s format strings, which call the object's __str__ and __repr__ methods respectively.

Example output demonstrates that !s produces a human‑readable string while !r yields a representation that includes quotes and can often be used to recreate the object.

>> 'foo {!s}'.format('bar')
'foo bar'
>>> 'foo {!r}'.format('bar')
"foo 'bar'"

The choice between __str__ and __repr__ depends on the object: if a class defines both, __str__ provides an informal, readable form, whereas __repr__ aims for an official representation that could be a valid Python expression.

When neither method is overridden, the default implementations inherited from object are used, with __repr__ typically returning a string like <ClassName object at 0x...> and __str__ delegating to __repr__.

These representations are primarily useful for debugging; __repr__ should be detailed and unambiguous, while __str__ should be concise and user‑friendly.

In format strings, the !s conversion flag selects the __str__ result and !r selects the __repr__ result, allowing explicit control over how values are formatted.