Recursive

A piece of code that can print itself

s = 's = %r\nprint(s%%s)'
print(s%s)

There are two builtin functions for turning an object into a string: str vs. repr:

• %s will call str, which is supposed to be a friendly, human readable string.
• %r will call repr, which 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

class Foo:

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

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

  def __str__(self):
    class_name = self.__class__.__name__
    return "%s(%s)" % (class_name, self.foo)

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

f = Foo('a')
print(f) # will call __str__ and output Foo(a) 

eval(repr(f)) == f # True