Overview. Advanced core Python.
Python tools. iterables, generators, classes...
This is all mtyn and we don't plan to use any of it. We include it because we got carried away.
We can define one-line lambda functions. in a streamlined way. To compute the ever-popular square of a number, we can define the function
square = lambda x: x**2We can now compute the square of 8 with square(8).
There are three functions that do implicit loops of different kinds:
- map. The statement
map(function, list)applies a function to all the elements of a list or similar object, producing another list of the same length. An example:
anylist = [2, 'Steelers', [1,2,3]]
m = map(type, anylist)
types = list(m)The last line is needed because map produces a "map object," which isn't all that helpful on its own. So we converted it to a list.
- reduce. The statement
reduce(function, list)applies a function to the elements of a list and produces a single value. The idea is that the calculations are connected. For example, we could compute a sum one element at a time:
from functools import *
numlist = [4, -2, 5]
sum = reduce(lambda x,y: x+y, numlist)The difference here is that the computations cannot be done separately, they're all connected.
- filter. Here we apply a logical condition to each element of a list or similar object and keep those for which the condition is true. Two examples:
numlist = [4, -2, 5]
f = filter(lambda x: x >= 0, numlist)
newlist = list(f)
anylist = [2, 'Steelers', [1,2,3]]
f = filter(lambda x: type(x) == str, anylist)
newlist = list(f)The first one has the answer newlist = [4, 5], the second newlist = ['Steelers']. Can you see why?
[?? talk about each one as an implicit loop?]
Some of us find this kind of syntax obscure and do our best to avoid it. But similar ideas show up in lots of places.
See also
http://people.duke.edu/~ccc14/sta-663/FunctionsSolutions.html#higher-order-functions
http://people.duke.edu/~ccc14/sta-663/FunctionsSolutions.html#anonymous-functions
Maybe some time.
http://blog.lerner.co.il/pythons-objects-and-classes-a-visual-guide/ http://blog.lerner.co.il/want-to-understand-pythons-comprehensions-think-like-an-accountant/ http://blog.lerner.co.il/quick-introduction-implementing-python-iterators/