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:
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.
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("<.*>", "\foo\and\so on\") = ['\foo\and\so on\']\ re.findall("<.*?>", "\foo\and\so on\") = ['\', '\', '\', '\']
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:
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