Lists

Lists, or arrays as I got to know them, are an integral part of programming. A list contains items. They can be indexed in with bracket notation my_list[0]. A slice can be taken using bracket notation as well spam[1:4], which goes up to but does not inlude the second index. If the first index is left out, it will start at the 0 index. Leaving both out copies the whole list. Regardless if part or all, slicing in this way always creates a new list.

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[:2]
['cat', 'bat']
>>> spam[1:]
['bat', 'rat', 'elephant']
>>> spam[:]
['cat', 'bat', 'rat', 'elephant']

The length of a list can be obtained by passing it to the len() method.

>>> len(spam)
4

The item at any index can be updated by assigment.

>>> spam[1] = 'aardvark'
>>> spam
['cat', 'aardvark', 'rat', 'elephant']

The + and * operators work on lists as well, similar to strings a pluls sign will append the two lists together, and the multiplier will copy the list and append to itself.

There is a del function that can unassign both simple variables and indexes in lists. In practice it is only really useful to delete from lists.

>>> del spam[2]
>>> spam
['cat', 'rat', 'elephant']

Lists are probably one of the most common data types to work with. The for item in <list> will iterate over every item in the array. But often it is desirable to know which index the loop is iterating on, and so len() comes into play as well as range() to provide an index iterator:

for index in range(len(someList)):
  print('Index ' + str(i) + ' in someList is: ' + someList[i])

There are also a pair of operators to help with knowing if an item is in a list or not (curious why not something more like indexOf). in and not in return a boolean for a conditional.

>>> spam = ['hello', 'hi', 'howdy', 'heyas']
>>> 'cat' in spam
False
>>> 'cat' not in spam
True

Tuple Unpacking

Tuple unpacking, aka multiple assignment, is a handy way to save some lines of code. As long as there is a variable for every value in the list, python can unpack it.

>>> cat = ['fat', 'gray', 'loud']
>>> size, color, disposition = cat

This allows for a bit nicer of a way to iterate on a list and get both the index and item at once.

for index, item in enumerate(supplies):

Randomicity in Lists

There is sometimes a need to randomly get one of the items from a list:

>>> import random
>>> pets = ['Dog', 'Cat', 'Moose']
>>> random.choice(pets)
'Dog'

Or maybe even to just shuffle the whole thing in place:

>>> random.shuffle(pets)
['Cat', 'Dog', 'Mouse']

Augmented Assignment

Augmented assignment is a shorthand to operate on a variable itself and save the new value to it.

Augmented assignment statement Equivalent assignment statement
spam += 1 spam = spam + 1
spam -= 1 spam = spam - 1
spam *= 1 spam = spam * 1
spam /= 1 spam = spam / 1
spam %= 1 spam = spam % 1

+ and * work for lists as well for concatenation and replication, respectively.

Methods vs Functions

Methods are similar to functions but are “called on” the item.
someList.index(item) is like other languages and will find the index of the item, an important difference here is that a ValueError: 'item' is not in list error will be thrown if it doesn’t exist. If duplicates in the list, the lower index will be returned.

Another handy one is someList.append(item), which appends the item to the end of the list.

someList.insert(indexInt, item) will insert the item into the specified index, shifting the rest of the items back.

someList.remove(item) will remove only the first item in the list it finds, it will raise a ValueError if the item wasn’t in the list to start with. The del statement is good to use when you know the index of the value you want to remove from the list. The remove() method is useful when you know the value you want to remove from the list.

spam.reverse() reverses the items of the list in place.

someList.sort() sorts numerically for ints/floats, asciibetical for strings; and it can take a keyword argument (kwarg) to sort in reverse someList.sort(reverse=True). It is not able to sort a list of mixed data types. Also, the sort is ASCIIbetical order rather than actual alphabetical order for sorting strings. This means uppercase letters come before lowercase letters. Therefore, the lowercase a is sorted so that it comes after the uppercase Z. ['Alice', 'Bob', 'Carol', 'ants', 'badgers', 'cats']

To sort by normal alphabetical oder, sort takes a keyword argument to treat each item as lowercase, but it won’t actually modify the list.

>>> spam = ['a', 'z', 'A', 'Z']
>>> spam.sort(key=str.lower)
>>> spam
['a', 'A', 'z', 'Z']

NOTE: These do not return the value of the new list as in some languages. They work in-place and return None.

Line Continuation

\ allows for code to run onto the next line if needed.

Sequence Data Types

The Python sequence data types include lists, strings, range objects returned by range(), and tuples.

Strings

The proper way to “mutate” a string is to use slicing and concatenation to build a new string by copying from parts of the old string.