Basic Python

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Python Basics

Useful information

list(xrange()) == range() –> In python3 range is the xrange of python2 (it is not a list but a generator)
The difference between a Tuple and a List is that the position of a value in a tuple gives it meaning but the lists are just ordered values. Tuples have structures but lists have an order.

Main operations

To raise a number you use: 3**2 (not 3^2)
If you do 2/3 it returns 1 because you are dividing two ints (integers). If you want decimals you should divide floats (2.0/3.0).
i >= j
i <= j
i == j
i != j
a and b
a or b
not a
float(a)
int(a)
str(d)
ord(“A”) = 65
chr(65) = ‘A’
hex(100) = ‘0x64’
hex(100)[2:] = ‘64’
isinstance(1, int) = True
“a b”.split(“ “) = [‘a’, ‘b’]
“ “.join([‘a’, ‘b’]) = “a b”
“abcdef”.startswith(“ab”) = True
“abcdef”.contains(“abc”) = True
“abc\n”.strip() = “abc”
“apbc”.replace(“p”,“”) = “abc”
dir(str) = List of all the available methods
help(str) = Definition of the class str
“a”.upper() = “A”
“A”.lower() = “a”
“abc”.capitalize() = “Abc”
sum([1,2,3]) = 6
sorted([1,43,5,3,21,4])

Join chars
3 * ’a’ = ‘aaa’
‘a’ + ‘b’ = ‘ab’
‘a’ + str(3) = ‘a3’
[1,2,3]+[4,5]=[1,2,3,4,5]

Parts of a list
‘abc’[0] = ‘a’
’abc’[-1] = ‘c’
’abc’[1:3] = ‘bc’ from [1] to [2]
“qwertyuiop”[:-1] = ‘qwertyuio’

Comments
# One line comment
“”“
Several lines comment
Another one
“”“

Loops

if a:
    #somethig
elif b:
    #something
else:
    #something

while(a):
    #comething

for i in range(0,100):
    #something from 0 to 99

for letter in "hola":
    #something with a letter in "hola"

Tuples

t1 = (1,’2,‘three’)
t2 = (5,6)
t3 = t1 + t2 = (1, ‘2’, ‘three’, 5, 6)
(4,) = Singelton
d = () empty tuple
d += (4,) –> Adding into a tuple
CANT! –> t1[1] == ‘New value’
list(t2) = [5,6] –> From tuple to list

List (array)

d = [] empty
a = [1,2,3]
b = [4,5]
a + b = [1,2,3,4,5]
b.append(6) = [4,5,6]
tuple(a) = (1,2,3) –> From list to tuple

Dictionary

d = {} empty
monthNumbers={1:’Jan’, 2: ‘feb’,’feb’:2}—> monthNumbers ->{1:’Jan’, 2: ‘feb’,’feb’:2}
monthNumbers[1] = ‘Jan’
monthNumbers[‘feb’] = 2
list(monthNumbers) = [1,2,’feb’]
monthNumbers.values() = [‘Jan’,’feb’,2]
keys = [k for k in monthNumbers]
a={‘9’:9}
monthNumbers.update(a) = {‘9’:9, 1:’Jan’, 2: ‘feb’,’feb’:2}
mN = monthNumbers.copy() #Independent copy
monthNumbers.get(‘key’,0) #Check if key exists, Return value of monthNumbers[“key”] or 0 if it does not exists

Set

In sets there are no repetitions
myset = set([‘a’, ‘b’]) = {‘a’, ‘b’}
myset.add(‘c’) = {‘a’, ‘b’, ‘c’}
myset.add(‘a’) = {‘a’, ‘b’, ‘c’} #No repetitions
myset.update([1,2,3]) = set([‘a’, 1, 2, ‘b’, ‘c’, 3])
myset.discard(10) #If present, remove it, if not, nothing
myset.remove(10) #If present remove it, if not, rise exception
myset2 = set([1, 2, 3, 4])
myset.union(myset2) #Values it myset OR myset2
myset.intersection(myset2) #Values in myset AND myset2
myset.difference(myset2) #Values in myset but not in myset2
myset.symmetric_difference(myset2) #Values that are not in myset AND myset2 (not in both)
myset.pop() #Get the first element of the set and remove it
myset.intersection_update(myset2) #myset = Elements in both myset and myset2
myset.difference_update(myset2) #myset = Elements in myset but not in myset2
myset.symmetric_difference_update(myset2) #myset = Elements that are not in both

Classes

The method in __It__ will be the one used by sort to compare if an object of this class is bigger than other

class Person(name):
	def __init__(self,name):
		self.name= name
		self.lastName = name.split(‘ ‘)[-1]
		self.birthday = None
 	def __It__(self, other):
		if self.lastName == other.lastName:
			return self.name < other.name
		return self.lastName < other.lastName #Return True if the lastname is smaller

	def setBirthday(self, month, day. year):
		self.birthday = date tame.date(year,month,day)
	def getAge(self):
		return (date time.date.today() - self.birthday).days


class MITPerson(Person):
	nextIdNum = 0	# Attribute of the Class
	def __init__(self, name):
		Person.__init__(self,name)
		self.idNum = MITPerson.nextIdNum  —> Accedemos al atributo de la clase
		MITPerson.nextIdNum += 1 #Attribute of the class +1

	def __it__(self, other):
		return self.idNum < other.idNum

map, zip, filter, lambda, sorted and one-liners

Map is like: [f(x) for x in iterable] –> map(tutple,[a,b]) = [(1,2,3),(4,5)]
m = map(lambda x: x % 3 == 0, [1, 2, 3, 4, 5, 6, 7, 8, 9]) –> [False, False, True, False, False, True, False, False, True]

zip stops when the shorter of foo or bar stops:

for f, b in zip(foo, bar):
    print(f, b)

Lambda is used to define a function
(lambda x,y: x+y)(5,3) = 8 –> Use lambda as simple function
sorted(range(-5,6), key=lambda x: x** 2) = [0, -1, 1, -2, 2, -3, 3, -4, 4, -5, 5] –> Use lambda to sort a list
m = filter(lambda x: x % 3 == 0, [1, 2, 3, 4, 5, 6, 7, 8, 9]) = [3, 6, 9] –> Use lambda to filter
reduce (lambda x,y: x*y, [1,2,3,4]) = 24

def make_adder(n):
	return lambda x: x+n
plus3 = make_adder(3)
plus3(4) = 7 # 3 + 4 = 7

class Car:
	crash = lambda self: print('Boom!')
my_car = Car(); my_car.crash() = 'Boom!'

mult1 = [x for x in [1, 2, 3, 4, 5, 6, 7, 8, 9] if x%3 == 0 ]

Exceptions

def divide(x,y):
	try:
		result = x/y
	except ZeroDivisionError, e:
		print “division by zero!” + str(e)
	except TypeError:
		divide(int(x),int(y))
	else:
		print “result i”, result
	finally
		print “executing finally clause in any case”

Assert()

If the condition is false the string will be printed in the screen

def avg(grades, weights):
	assert not len(grades) == 0, 'no grades data'
	assert len(grades) == 'wrong number grades'

Generators, yield

A generator, instead of returning something, it “yields” something. When you access it, it will “return” the first value generated, then, you can access it again and it will return the next value generated. So, all the values are not generated at the same time and a lot of memory could be saved using this instead of a list with all the values.

def myGen(n):
	yield n
	yield n + 1

g = myGen(6) –> 6
next(g) –> 7
next(g) –> Error

Regular Expresions

import re
re.search(“\w”,“hola”).group() = “h”
re.findall(“\w”,“hola”) = [‘h’, ‘o’, ‘l’, ‘a’]
re.findall(“\w+(la)”,“hola caracola”) = [‘la’, ‘la’]

Special meanings:
. –> Everything
\w –> [a-zA-Z0-9_]
\d –> Number
\s –> WhiteSpace char[ \n\r\t\f]
\S –> Non-whitespace char
^ –> Starts with
$ –> Ends with
+ –> One or more
* –> 0 or more
? –> 0 or 1 occurrences

Options:
re.search(pat,str,re.IGNORECASE)
IGNORECASE
DOTALL –> Allow dot to match newline
MULTILINE –> Allow ^ and $ to match in different lines

re.findall(“<.*>”, “<b>foo</b>and<i>so on</i>”) = [‘<b>foo</b>and<i>so on</i>’]
re.findall(“<.*?>”, “<b>foo</b>and<i>so on</i>”) = [‘<b>’, ‘</b>’, ‘<i>’, ‘</i>’]

IterTools
product
from itertools import product –> Generates combinations between 1 or more lists, perhaps repeating values, cartesian product (distributive property)
print list(product([1,2,3],[3,4])) = [(1, 3), (1, 4), (2, 3), (2, 4), (3, 3), (3, 4)]
print list(product([1,2,3],repeat = 2)) = [(1, 1), (1, 2), (1, 3), (2, 1), (2, 2), (2, 3), (3, 1), (3, 2), (3, 3)]

permutations
from itertools import permutations –> Generates combinations of all characters in every position
print list(permutations([‘1’,‘2’,‘3’])) = [(‘1’, ‘2’, ‘3’), (‘1’, ‘3’, ‘2’), (‘2’, ‘1’, ‘3’),… Every posible combination
print(list(permutations(‘123’,2))) = [(‘1’, ‘2’), (‘1’, ‘3’), (‘2’, ‘1’), (‘2’, ‘3’), (‘3’, ‘1’), (‘3’, ‘2’)] Every possible combination of length 2

combinations
from itertools import combinations –> Generates all possible combinations without repeating characters (if “ab” existing, doesn’t generate “ba”)
print(list(combinations(‘123’,2))) –> [(‘1’, ‘2’), (‘1’, ‘3’), (‘2’, ‘3’)]

combinations_with_replacement
from itertools import combinations_with_replacement –> Generates all possible combinations from the char onwards(for example, the 3rd is mixed from the 3rd onwards but not with the 2nd o first)
print(list(combinations_with_replacement(‘1133’,2))) = [(‘1’, ‘1’), (‘1’, ‘1’), (‘1’, ‘3’), (‘1’, ‘3’), (‘1’, ‘1’), (‘1’, ‘3’), (‘1’, ‘3’), (‘3’, ‘3’), (‘3’, ‘3’), (‘3’, ‘3’)]

Decorators

Decorator that size the time that a function needs to be executed (from here):

from functools import wraps
import time
def timeme(func):
  @wraps(func)
  def wrapper(*args, **kwargs):
    print("Let's call our decorated function")
    start = time.time()
    result = func(*args, **kwargs)
    print('Execution time: {} seconds'.format(time.time() - start))
    return result
  return wrapper

@timeme
def decorated_func():
  print("Decorated func!")

If you run it, you will see something like the following:

Let's call our decorated function
Decorated func!
Execution time: 4.792213439941406e-05 seconds

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