A list is a container that holds a sequence of related pieces of information.
The shortest list is an empty list, just 2 square brackets:
members = []
Lists can hold any Python values, separated by commas:
members = ["Pamela", "Tinu", "Brenda", "Kaya"]
ages_of_kids = [1, 2, 7]
prices = [79.99, 49.99, 89.99]
digits = [2//2, 2+2+2+2, 2, 2*2*2]
remixed = ["Pamela", 7, 79.99, 2*2*2]
Use the global len() function
to find the length of a list.
attendees = ["Tammy", "Shonda", "Tina"]
print(len(attendees)) # 3
num_of_attendees = len(attendees)
print(num_of_attendees)
🤔 What could go wrong with storing the length?
Each list item has an index, starting from 0.
letters = ['A', 'B', 'C']
# Index: 0 1 2
Access each item by putting the index in brackets:
letters[0] # 'A'
letters[1] # 'B'
letters[2] # 'C'
letters[3] # 🚫 Error!
curr_ind = 1
letters[curr_ind] # 'B'
Negative indices are also possible:
letters[-1] # 'C'
letters[-2] # 'B'
letters[-4] # 🚫 Error!
It's also possible to use a function from the operator module:
from operator import getitem
getitem(letters, 0)
Add two lists together using the + operator:
boba_prices = [5.50, 6.50, 7.50]
smoothie_prices = [7.00, 7.50]
all_prices = boba_prices + smoothie_prices
Or the add function:
from operator import add
boba_prices = [5.50, 6.50, 7.50]
smoothie_prices = [7.00, 7.50]
all_prices = add(boba_prices, smoothie_prices)
Concatenate the same list multiple times using the * operator:
boba_prices = [5.50, 6.50, 7.50]
more_boba = boba_prices * 3
Or the mul function:
from operator import mul
boba_prices = [5.50, 6.50, 7.50]
more_boba = mul(boba_prices, 3)
All together now:
digits = [1, 9, 8, 4]
together = [6, 2, 4] + digits * 2 # [6, 2, 4, 1, 9, 8, 4, 1, 9, 8, 4]
together = add([2, 7], mul(digits, 2))
Since Python lists can contain any values, an item can itself be a list.
gymnasts = [ ["Brittany", 9.15, 9.4, 9.3, 9.2],
["Lea", 9, 8.8, 9.1, 9.5],
["Maya", 9.2, 8.7, 9.2, 8.8] ]
gymnasts? 3
gymnasts[0]? 5
gymnasts = [
["Brittany", 9.15, 9.4, 9.3, 9.2],
["Lea", 9, 8.8, 9.1, 9.5],
["Maya", 9.2, 8.7, 9.2, 8.8]
]
Access using bracket notation, with more brackets as needed:
gymnasts[0] # ["Brittany", 9.15, 9.4, 9.3, 9.2]
gymnasts[0][0] # "Brittany"
gymnasts[1][0] # "Lea"
gymnasts[1][4] # 9.5
gymnasts[1][5] # 🚫 IndexError!
gymnasts[3][0] # 🚫 IndexError!
Use the in operator to test if value is inside a container:
digits = [2, 8, 3, 1, 8, 5, 3, 0, 7, 1]
1 in digits # True
3 in digits # True
4 in digits # False
not (4 in digits) # True
The for loop syntax:
for <value> in <sequence>:
<statement>
<statement>
The for loop provides a cleaner way to write many while loops,
as long as they are iterating over some sort of sequence.
def count(s, value):
total = 0
for element in s:
if element == value:
total = total + 1
return total
for <name> in <expression>:
<suite>
<expression>, which must yield an iterable value (a sequence)
<name> to that element in the current frame
<suite>
gymnasts = [
["Brittany", 9.15, 9.4, 9.3, 9.2],
["Lea", 9, 8.8, 9.1, 9.5],
["Maya", 9.2, 8.7, 9.2, 8.8]
]
Use a nested for-in loop:
for gymnast in gymnasts:
for data in gymnast:
print(data, end="|")
Remember what type of data is being stored in the loop variable!
pairs = [[1, 2], [2, 2], [3, 2], [4, 4]]
same_count = 0
for x, y in pairs:
if x == y:
same_count = same_count + 1
Each name is bound to a value, like in multiple assignment.
A range represents a sequence of integers.
... -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5...
range(-2, 3)
If just one argument, range starts at 0 and ends just before it:
for num in range(6):
print(num) # 0, 1, 2, 3, 4, 5
If two arguments, range starts at first and ends just before second:
for num in range(1, 6):
print(num) # 1, 2, 3, 4, 5
A way to create a new list by "mapping" an existing list.
Short version:
[<map exp> for <name> in <iter exp>]
odds = [1, 3, 5, 7, 9]
evens = [(num + 1) for num in odds]
Long version (with filter):
[<map exp> for <name> in <iter exp> if <filter exp>]
temps = [60, 65, 71, 67, 77, 89]
hot = [temp for temp in temps if temp > 70]
[<map exp> for <name> in <iter exp> if <filter exp>]
<iter exp>:
<name> to that element in the new frame from step 1
<filter exp> evaluates to a true value, then add the value of <map exp> to the result list
letters = ['a', 'b', 'c', 'd', 'e', 'f', 'm', 'n', 'o', 'p']
word = [letters[i] for i in [3, 4, 6, 8]]
def divisors(n):
"""Returns all the divisors of N.
>>> divisors(12)
[1, 2, 3, 4, 6]
"""
def divisors(n):
"""Returns all the divisors of N.
>>> divisors(12)
[1, 2, 3, 4, 6]
"""
return [x for x in range(1, n) if n % x == 0]
def front(s, f):
"""Return S but with elements chosen by F at the front.
>>> front(range(10), lambda x: x % 2 == 1) # odds in front
[1, 3, 5, 7, 9, 0, 2, 4, 6, 8]
"""
def front(s, f):
"""Return S but with elements chosen by F at the front.
>>> front(range(10), lambda x: x % 2 == 1) # odds in front
[1, 3, 5, 7, 9, 0, 2, 4, 6, 8]
"""
return [e for e in s if f(e)] + [e for e in s if not f(e)]
Representing data:
'2,400' '2.400' '1.2e-5'
Representing language:
"""Se lembra quando a gente
Chegou um dia a acreditar
Que tudo era pra sempre
Sem saber
Que o pra sempre sempre acaba"""
Representing programs:
'curry = lambda f: lambda x: lambda y: f(x, y)'
Single quoted strings and double quoted strings are equivalent:
'您好, I am a string, hear me roar 🦁!'
"I've got an apostrophe"
Multi-line strings automatically insert new lines:
"""The Zen of Python
claims, Readability counts.
Read more: import this."""
# 'The Zen of Python\nclaims, Readability counts.\nRead more: import this.'
The \n is an escape sequence
signifying a line feed.
alfabeto = 'abcdefghijklmnñopqrstuvwxyz'
len(alfabeto) # 27
alfabeto[14] + "andu" # ñandu
alfabeto + ' ¡Ya conoces el ABC!'
A single-character string is the same as the character itself.
initial = 'P'
initial[0] == initial # True
The in operator will match substrings:
'W' in 'Where\'s Waldo' # True
'Waldo' in 'Where\'s Waldo' # True