.. _lambda_functions:

--------------------------------
Lambda functions
--------------------------------

Some functions can be easily defined in a single line of code, and it is
sometimes useful to be able to define a function "on the fly" using ``lambda``
notation. ``lambda`` is a special way of creating small, single-line
functions. The format of the lambda is::

  lambda <arguments>: <single exprsession>


To define a function that returns ``2*x`` for any input ``x``, rather
than::

    def f(x):
        return 2*x


Although lambda functions are *anonymous functions*, this doesn't
mean that you can't give it a name. We can define ``f`` via::

  >>> f = lambda x: 2*x

A lambda expression can also have a list::


  >>> f = lambda: [x**2 for x in range(10)]
  >>> f()
  [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

  
You can also define functions of more than one variable, e.g.::

  >>> g = lambda x,y: 2*(x+y)

A lambda function can be called after it is defined::

  >>> (lambda x, y=10: 2*x + y) (42)
  94

A more complicated example which uses a lambda function as a keyword
argument (i.e., ``f=lambda...``) in another function ``func``::

  >>> def func(vals, f=lambda x: sum(x)/len(x)):
          f(vals)
  >>> func([1,2,3,4,5])
  3
  

    
.. note::
   The lambda notation is one of the built-in Python keywords :ref:`ch03-python-keywords`,
   which means importing NumPy is *not* needed to use it.

.. note::
   One thing you notice here is that the keyword argument ``f=...`` is
   supplied optionally. If there are multiple keyword arguments, they
   may be called in any order and are not restricted to the order in
   which they were originally defined.