Python Lists Quick Reference

Construction
Accessing Data
Modifying
Sorting
Deep Copy
Equality and Identity
List Comprehensions

1. Construction



In [30]:

    
#create an empty list
empty_list=[]
empty_list=list()
simpsons = ['homer', 'marge', 'bart']

2. Accessing Data



In [31]:

    
simpsons[0]









    Out[31]:





'homer'



In [32]:

    
len(simpsons)









    Out[32]:





3



In [33]:

    
# counts the number of instances
simpsons.count('bart')









    Out[33]:





1



In [34]:

    
# returns index of the first reference
simpsons.index('marge')









    Out[34]:





1

3. Modifying



In [35]:

    
#append to end
simpsons.append('lisa')
simpsons









    Out[35]:





['homer', 'marge', 'bart', 'lisa']



In [36]:

    
#append multiple elments to end
simpsons.extend(['itchy','scratchy'])
simpsons









    Out[36]:





['homer', 'marge', 'bart', 'lisa', 'itchy', 'scratchy']



In [37]:

    
#insert at index (shifts all elements to the right)
simpsons.insert(0, 'maggie')
simpsons









    Out[37]:





['maggie', 'homer', 'marge', 'bart', 'lisa', 'itchy', 'scratchy']



In [38]:

    
#remove element 0 and return it
simpsons.pop(0)









    Out[38]:





'maggie'



In [39]:

    
#remove element at 0 (does not return it)
del simpsons[0]
simpsons









    Out[39]:





['marge', 'bart', 'lisa', 'itchy', 'scratchy']



In [40]:

    
#search for first instance and remove it
simpsons.remove('bart')
simpsons









    Out[40]:





['marge', 'lisa', 'itchy', 'scratchy']



In [41]:

    
#replace elmeent
simpsons[0]='krusty'
simpsons









    Out[41]:





['krusty', 'lisa', 'itchy', 'scratchy']



In [42]:

    
#conctonate lists (slower then 'extend' method)
neighbors = simpsons + ['ned','rod','todd']
neighbors









    Out[42]:





['krusty', 'lisa', 'itchy', 'scratchy', 'ned', 'rod', 'todd']

Setwise list manipulations:



In [43]:

    
neighbors * 2









    Out[43]:





['krusty',
 'lisa',
 'itchy',
 'scratchy',
 'ned',
 'rod',
 'todd',
 'krusty',
 'lisa',
 'itchy',
 'scratchy',
 'ned',
 'rod',
 'todd']

4. Sorting

Sort a list in place (modifies but does not return the list):



In [44]:

    
simpsons.sort()
simpsons









    Out[44]:





['itchy', 'krusty', 'lisa', 'scratchy']



In [45]:

    
#reverse sort
simpsons.sort(reverse=True)
simpsons









    Out[45]:





['scratchy', 'lisa', 'krusty', 'itchy']



In [46]:

    
#sort by a key
simpsons.sort(key=len)
simpsons









    Out[46]:





['lisa', 'itchy', 'krusty', 'scratchy']

Return a sorted list (does not modify the original list):



In [47]:

    
sorted(simpsons)









    Out[47]:





['itchy', 'krusty', 'lisa', 'scratchy']



In [48]:

    
sorted(simpsons, reverse=True, key=len)









    Out[48]:





['scratchy', 'krusty', 'itchy', 'lisa']

Insert into an already sorted list, and keep it sorted:



In [49]:

    
num = [10, 20, 40, 50]
from bisect import insort
insort(num, 30)
num









    Out[49]:





[10, 20, 30, 40, 50]

5. Deep Copy



In [50]:

    
new_num = num[:]
new_num=list(num)
new_num









    Out[50]:





[10, 20, 30, 40, 50]

6. Equality and Identity



In [51]:

    
# Identity compare: are they referencing the same object?
num is new_num









    Out[51]:





False



In [52]:

    
# Equality compare: do they have the same data?
num == new_num









    Out[52]:





True

7. List Comprehensions



In [2]:

    
mylist = [1, 4, -5, 10, -7, 2, 3, -1]
[n for n in mylist if n > 0]









    Out[2]:





[1, 4, 10, 2, 3]



In [3]:

    
[n for n in mylist if n < 0]
[-5, -7, -1]









    Out[3]:





[-5, -7, -1]



In [4]:

    
import math
[math.sqrt(n) for n in mylist if n > 0]









    Out[4]:





[1.0, 2.0, 3.1622776601683795, 1.4142135623730951, 1.7320508075688772]



In [5]:

    
#clip negative values
[n if n > 0 else 0 for n in mylist]









    Out[5]:





[1, 4, 0, 10, 0, 2, 3, 0]



In [5]:

    
# nested loops flattened into one list
alphas = 'abc'
digits = '123'
correct = ['a1', 'a2','a3','b1','b2','b3','c1','c2','c3']

answer = [alpha+digit for alpha in alphas for digit in digits]
answer == correct









    Out[5]:





True