A list of numbers

Documentation

1. list in the Python Glossary
2. Lists and Tools for working with lists in the Python Tutorial
3. Lists in the Python Standard Library
4. Lists in the Python Language Reference
5. array.array is like a list of numbers that are all of the same type.
   1. array.array in the Python Standard Library.
   2. array.array in Tools for working with lists

A list is an example of a (mutable) sequence.

1. container in the Python Language Reference
2. sequence in the Python Glossary
3. Sequences in the Python Language Reference
4. Tuples and sequences and Comparing sequences and other types in the Python Tutorial
5. Sequence types in the Python Standard Library
6. A list can do all the common sequence operations and all the mutable sequence operations.

A sequence is also an example of an iterable.

1. iterable in the Python Glossary
2. Iterators in the Python Tutorial
3. An iterable object has either a __iter__ method or a __getitem__ method.

How not to program

If we had thousands of numbers, we would have to invent thousands of names.

list1.py

A year has 12 months.
The first month has 31 days.
The second month has 28 days.
The third month has 28 days.
The last month month has 31 days.
The next-to-last month month has 30 days.
The third-to-last month month has 31 days.

A list of numbers

list2.py

We have just mass-produced 12 variables, named months[0], months[1], months[2], months[3], …, months[9], months[10], months[11]. These variables are called the items contained by the list. The integer in the [square brackets] is called the index or subscript. Indexing a list is just like indexing a string.

The variable months[0] contains the int 31.
The variable months[1] contains the int 28.
The variable months[2] contains the int 31.
The variable months[11] contains the int 31.

Warning: there is no variable months[12]. That’s because the indices of a list start at 0, just like the indices of a string, and in this example there are only 12 variables.

A year has 12 months.
The first month has 31 days.
The second month has 28 days.
The third month has 31 days.
The twelfth month has 31 days.
The last month has 31 days.
The next-to-last month has 30 days.
The third-to-last month has 31 days.

Things to try

1. What happens if you try to access the non-existent variable months[12]?

   print(f"The thirteenth month has {months[12]} days.")
   

   Traceback (most recent call last):
     File "/Users/myname/python/list2.py", line 31, in <module>
       print("The thirteenth month has", months[12], "days.")
   IndexError: index out of range
   

   print(f"The third and a halfth month has {months[2.5]} days.")
   

   Traceback (most recent call last):
     File "/Users/myname/python/Untitled.py", line 31, in <module>
       print("The third and a halfth month has", months[2.5], "days.")
   TypeError: list indices must be integers or slices, not float
   



2. We saw here that a string is an immutable sequence. A list, however, is a mutable sequence.

   months[1] = 29   #February
   print(f"The second month has {months[1]} days.")
   

   The second month has 29 days.
   

   This gives us a way to change the characters of a string.

   s = "counterclockwise"
   listOfCharacters = list(s)     #Convert the string of chars into a list of chars.
   listOfCharacters[12] = "W"     #A list, unlike a string, is mutable.
   s = "".join(listOfCharacters)  #Convert the list back into a string.
   print(s)
   

   counterclockWise