Fun with funS

Functional programming in Python

Paweł Pamuła

Overview

• Why do we bother?
• Is Python a functional language?
• Does Python have functional features?
• Let's see how do they work!
  

Programming paradigms

Functional Concepts

Advantages of functional approach

Python does not promote functional approach!

I have never considered Python to be heavily influenced by functional languages, no matter what people say or think.

However, earlier on, it was clear that users wanted to do much more with lists and functions.

Functional features in python

• functions as first-class objects
• lambdas
• lazy evaluation
• map, filter, reduce
• functools & itertools
  

First class objects

>>> def forall(fun, val):
...     output = []
...     for v in val:
...             output.append(fun(v))
...     return output
... 

>>> def double(x):
...     return 2*x
... 

>>> forall(double, [1, 2, 3, 4])
[2, 4, 6, 8]

LAMBDA FUNCTIONS

 >>> reverse = lambda s: s[::-1]
 >>> reverse('abc')
 'cba'

>>> forall(lambda s: s[::-1], ['abc', 'kajak'])
['cba', 'kajak']

Currying

>>> def log(level, message):
...     print "[%s] %s" % (level, message)
... 

>>> log("error", "It's gonna explode!")
[error] It's gonna explode!

CURRYING

from functools import partial

def log(level, message):
	print "[%s] %s" % (level, message)

levels = ['info', 'warning', 'error', 'debug']
funs = dict((item, partial(log, item)) for item in levels)
funs['info']("That's a very important information")
print funs

[info] That's a very important information

Lazy evaluation

• Evaluation strategy which delays the evaluation of an expression until its value is needed
• Enables construction of potentially infinite structures
• Increases performance
  

lazy evaluation

>>> 1 or 1/0
1

>>> import time
>>> print ['some text', time.sleep(3)][0]

MAP

• Function map takes function and collection as a parameter
• Applies function to every element

import os
files = ['file1.pdf', 'file2.txt']
filename = lambda f: os.path.splitext(f)[0]
modules = map(filename, files)

['file1', 'file2']

MAP & CURRYING

Sum of integers

 def isum(a, b):
	return sum(range(a, b+1))

Sum of squares

def ssum(a, b):
	return sum(map(lambda x: x**2, range(a, b+1)))

Sum of roots

 import math def rsum(a, b):
	return sum(map(lambda x: math.sqrt(x), range(a, b+1)))

MAP & CURRYING

import math

def general_sum(f):
	def inner(a, b):
		return sum(map(f, range(a, b+1)))
	#returns function!
	return inner

log_sum = general_sum(math.log)

print log_sum(1, 10), ' should be more or less equal to ',             math.log(reduce(lambda x, y: x*y, range(1, 11)))

 15.1044125731  should be more or less equal to  15.1044125731

REDUCE

reduce

 >>> f = lambda a,b: "f(%s,%s)" % (a,b)
>>> reduce(f, '1234')
f(f(f(1,2),3),4)

REDUCE, ANOTHER EXAMPLE

>>> mul = lambda x, y: 10*x + y
>>> reduce(mul, [1, 4, 1, 0])
1410

ANOTHER REDUCE EXAMPLE

 def gcd(a, b):
	while b:
		a, b = b, a % b
	return a

def lcm(a, b):
	return a * b // gcd(a, b)

def lcmm(*args):
	return reduce(lcm, args)

print lcmm(10, 17, 4)

LAST BUT NOT LEAST: FILTER

• Accepts function & iterable as an argument
• Returns (filtered) list
• Example is superfluous :)
  

BONUS QUESTION

How are parameters passed to function in Python?

def f(x):
	x[0] = 10

def g(x):
	x = [4, 5, 6, 7]

def h(some_string):
	some_string = 'New string'

x = [0, 1, 2, 3]
f(x)
h(x)

s = 'Some string'
h(s)

Next part of the presentation(?)

• higher-order functions
• functools, itertools
• closures
• decorators
• ...
  

Thank you for your attention!

Paweł Pamuła

pawel.pamula@gmail.com

@github: PawelPamula