Lambda
======

.. code-block:: ipython

    In [171]: f = lambda x, y: x+y
    In [172]: f(2,3)
    Out[172]: 5

Content of function can only be an expression -- not a statement

Anyone remember what the difference is?

Called "Anonymous": it doesn't get a name.

.. nextslide::

It's a python object, it can be stored in a list or other container

.. code-block:: ipython

    In [7]: l = [lambda x, y: x+y]
    In [8]: type(l[0])
    Out[8]: function


And you can call it:

.. code-block:: ipython

    In [9]: l[0](3,4)
    Out[9]: 7

Functions as first class objects
---------------------------------

You can do that with "regular" functions too:

.. code-block:: ipython

    In [12]: def fun(x,y):
       ....:     return x+y
       ....:
    In [13]: l = [fun]
    In [14]: type(l[0])
    Out[14]: function
    In [15]: l[0](3,4)
    Out[15]: 7


A bit more about lambda
------------------------

It is very useful for specifying sorting as well:

.. code-block:: ipython

    In [55]: lst = [("Chris","Barker"), ("Fred", "Jones"), ("Zola", "Adams")]

    In [56]: lst.sort()

    In [57]: lst
    Out[57]: [('Chris', 'Barker'), ('Fred', 'Jones'), ('Zola', 'Adams')]

    In [58]: lst.sort(key=lambda x: x[1])

    In [59]: lst
    Out[59]: [('Zola', 'Adams'), ('Chris', 'Barker'), ('Fred', 'Jones')]

lambda in keyword arguments
---------------------------

.. code-block:: ipython

    In [186]: l = []
    In [187]: for i in range(3):
        l.append(lambda x, e=i: x**e)
       .....:
    In [189]: for f in l:
        print(f(3))
    1
    3
    9

Note when the keyword argument is evaluated: this turns out to be very handy!