Basic Python

Python Basics

Usefull information

It is an interpreted language 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 a meaning but the lists are just ordered values. Tuples have structures, lists have order

Main operations

To raise a number you should do: 3**2 (it isn't 3^2) If you do 2/3 it returns 1 because you are dividing two ints. 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 availble 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 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 the sets there are not 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 in order 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 by 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 posible combination of lenght 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