what's the meaning of %r in python

Question

what's the meaning of %r in the following statement?

print '%r' % (1)

I think I've heard of %s, %d, and %f but never heard of this.

Answer 1

It calls repr() on the object and inserts the resulting string.

Answer 2

It prints the replacement as a string with repr().

Answer 3

See String Formatting Operations in the docs. Notice that %s and %d etc, might work differently to how you expect if you are used to the way they work in another language such as C.

In particular, %s also works well for ints and floats unless you have special formatting requirements where %d or %f will give you more control.

Answer 4

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

Python's documentation, describing these methods and their related functions:

http://docs.python.org/reference/datamodel.html#object.__repr__