collections

lists

list_items = [ "value0" , "value1" , "value2" ]

list allow dublicate values

lists are indexed i.e. ordered

lists values can be changed

tuples

tuples = ( "value0" , "value1" , "value2" )

tuples allow dublicate values

tuples are indexed i.e. ordered

tuples are immutable

sets

set_items = { "valueY" , "valueX" , "valueZ" }

set_items = { "valueX" , } sic!

sets are always unique

sets are not indexed i.e. unordered

sets are immutable (items, however, can be deleted, and added)

single sets end with a comma

dictionaries

dictionary_items = { "keyA" : "valueA" , "keyB" : "valueB" }

dictionaries are always unique

a dictionary's items are key-value-pairs

dictionaries are not indexed, but ordered (the key works like a named index)

help

help(list) shows methods etc.

syntax

long_list = short_list_1 + short_list_2

sum(my_list)

min(my_list)

max(my_list)

len(my_list)

my_list = [] #creates an empty list of

my_list[n] = new_value #replaces the item on position n on the list with a new value

my_list.extend(["A","B"]) #add the elements A and B to my_list

methods

https://www.w3schools.com/python/python_lists_methods.asp

.append(value) #adds a value to the list as an additional item

.count() #counts elements in a list

  .count("e") #counts elements "e" in the list's elements

.extend() #combines two lists

.index() #gives the index number of an element in a list, or the position of a character in a string

  .index(value) #gvie the index of the value

.insert(n,new_value) #adds a new value to the list at position n

.pop() #deletes the last item from the list

  .pop(n) #deletes the last item at position n from the list

  .pop("name") #detes the item "name" from the list.reverse() #reverses a list

.sort() #sorts a list, and stores it as itself

sorted() #sorts a set and returns a list

.unique() #makes a list unique distinct list of values

tuple() #converts to a tuple

unpacking

tuple unpacking is also called tuple expansion

var1, var2 = my_tuple(0) #assign each tuple pair of the first tuple to a variable

syntax examples

for var1, var2 in my_tuples:

  print(var1)

  print(var2)

methods

https://www.w3schools.com/python/python_tuples_methods.asp

.items #gives the keys and values as tuples in a list

.keys #give the keys of a dictionary, not the values

.values #gives the values of a dictionary, not the keys

var = set()

set() #can cast a list to a set

sorted() #sorts a set and returns a list

good to know

search works very fast on sets

double values

double values will appear only once in the print() of a set

sets are used to sort out duplicates

methods

https://www.w3schools.com/python/python_sets.asp

.add #adds an element to a set, if the element does not exist already in the set

.discard() #removes elements from a set

.difference #shows differnce between a set and a target set

.intersection() #intersection of sets

.pop() #deletes an element from a set

.union() #union of sets

.update() #adds to a set

structure

{ item1 , item2 , … }

{ "key1": value1 , "key2": value2, …  }

double keys

double keys are not possible, of equal keys only the latest gives the valid value

nesting

dictionaries are sometime nested inside other dictionaries to create an efficient data structure

handling

my_dictionary["my_key"] #accesses the value of the key "my_key"

del my_dictionary["my_key"] deletes the key and its value from the dictionary

in #gives True if a key is in a dictionary

my_dictionary["my_key"] = value #adds or changes a dictionary

nesting

dicationary can be nested

my_dictionary["my_key"]["my_subkey"] #accesses the value of the subdictionary inside the dictionary

for loop

for key, value in my_dictionary.items():

  print(key, value)

methods

https://www.w3schools.com/python/python_dictionaries_methods.asp

.get("my_key") #gets a value avoiding error messages

.items #gives the keys and values as tuples in a list

.keys #gives the keys of a dictionary, not the values

.pop("my_key") deletes the key and its value from the dictionary

.update({"my_key": value}) #updates or add to a dictionary

.values #gives the values of a dictionary, not the keys

syntax snippets

"my_key" in my_dictionary #is True, if the key is part of the dictionary

print(my_dictionary["my_key"]) #gives the value to "my_key" in my_dictionary

del_value = my_dictionary.pop("my_key") #deletes the key and its value from the dictionary saves the value in a variable

capitals = {'Australia':'Canberra', 'Belgium':'Brussels', 'Finland':'Helsinki'}

for key,val in capitals.items():

  print(key,val)

print([squirrel for squirrel in squirrels_by_park["Union Square Park"] if "Cinnamon" in squirrel["primary_fur_color"]])

numpy

the slice notations works slightly different as it handles dimensions viz. row and columns of arrays

numpy array

cf. iterations

index slice notation

start:stop

[start:end] with start included, but end is excluded!

my_list = [0, 1, 2, 3, 4, 5, 6, 7 ,8]

print(len(my_list)) #output 9

print(my_list) #output [0, 1, 2, 3, 4, 5, 6, 7 ,8]

print(my_list[:]) #output [0, 1, 2, 3, 4, 5, 6, 7 ,8]

print(my_list[:2]) #output [0, 1]

print(my_list[6]) #output 6

print(my_list[6:]) #output [6, 7 ,8]

print(my_list[2:6]) #output [2, 3, 4, 5]

print(my_list[6:2]) #output []

print(my_list[-1]) #output 8

print(my_list[-1:6]) #output []

print(my_list[3:-2]) #output [3, 4, 5, 6]

start:stop:step

my_list = [0, 1, 2, 3, 4, 5, 6, 7 ,8]

print(len(my_list)) #output 9

print(my_list[::2]) #output [0, 2, 4, 6, 8]

print(my_list[1::3]) #output [1, 4, 7]

print(my_list[1:7:3]) #output [1, 4]

the collections module holds more advances data containers viz. collections