What is the meaning of %r?

python, repr

Solution

Background:

In Python, there are two builtin functions for turning an object into a string: `str` vs. `repr`. `str` is supposed to be a friendly, human readable string. `repr` is supposed to include detailed information about an object's contents (sometimes, they'll return the same thing, such as for integers). By convention, if there's a Python expression that will eval to another object that's ==, `repr` will return such an expression e.g.

>>> print repr('hi')
'hi'  # notice the quotes here as opposed to...
>>> print str('hi')
hi

If returning an expression doesn't make sense for an object, `repr` should return a string that's surrounded by < and > symbols e.g. `<blah>`.

To answer your original question:

`%s` <-> `str` `%r` <-> `repr`

In addition:

You can control the way an instance of your own classes convert to strings by implementing `__str__` and `__repr__` methods.

class Foo:

  def __init__(self, foo):
    self.foo = foo

  def __eq__(self, other):
    """Implements ==."""
    return self.foo == other.foo

  def __repr__(self):
    # if you eval the return value of this function,
    # you'll get another Foo instance that's == to self
    return "Foo(%r)" % self.foo

Problem

Original source