python - What does !r do in str() and repr()?

Question

According to the Python 2.7.12 documentation:

!s (apply str()) and !r (apply repr()) can be used to convert the value before it is formatted.

>>> import math
>>> print 'The value of PI is approximately {}.'.format(math.pi)
The value of PI is approximately 3.14159265359.
>>> print 'The value of PI is approximately {!r}.'.format(math.pi)
The value of PI is approximately 3.141592653589793.

Interestingly, the converted value is the output of repr(), rather than str().

>>> str(math.pi)
'3.14159265359'
>>> repr(math.pi)
'3.141592653589793'

So what does "convert the value" mean here? Making it less human-readable?

Answer 1

In order to format something in a string, a string representation of that something must first be created. "convert the value" is basically talking about how the string representation is to be constructed. In python, there are two fairly natural choices to get a string representation of something ... str and repr. str is generally a little more human friendly, repr is generally more precise. Perhaps the official documentation is the best place to go looking for the difference:

object.__repr__(self)

• Called by the repr() built-in function to compute the “official” string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <...some useful description...> should be returned. The return value must be a string object. If a class defines __repr__() but not __str__(), then __repr__() is also used when an “informal” string representation of instances of that class is required.

• This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.

object.__str__(self)

• Called by str(object) and the built-in functions format() and print() to compute the “informal” or nicely printable string representation of an object. The return value must be a string object.

• This method differs from object.__repr__() in that there is no expectation that __str__() return a valid Python expression: a more convenient or concise representation can be used.

• The default implementation defined by the built-in type object calls object.__repr__().

In str.format, !s chooses to use str to format the object whereas !r chooses repr to format the value.

The difference can easily be seen with strings (as repr for a string will include outer quotes).:

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

What the difference between these two methods really depends critically on the objects being formatted. For many objects (e.g. those that don't override the __str__ method), there will be no difference in the formatted output.