List Methods

This is a list of list methods. In it, L is the list to which the method is applied, M is a list, x is an element to be added to, looked for, or removed from, a list, and i is an index of a list element.

Operation	Description
`L.append(x)`	Append element `x` to `L`
`L.count(x)`	Count the number of times `x` occurs in `L`
`L.extend(M)`	Extend `L` by adding the elements of `M` at the end
`L.index(x)`	Return the index of the first occurrence of `x` in `L`; `ValueError` exception if `x` not in `L`
`L.insert(i,x)`	Insert `x` at position `i` in `L`
`L.pop()`	Remove and return the last element of `L`
`L.pop(i)`	Remove and return the element of `L` at position `i`; `IndexError` exception if `i` out of range
`L.remove(x)`	Remove the first occurrence of `x` from `L`; `ValueError` exception if `x` not in `L`
`L.reverse()`	Reverse `L` in place (does not make a copy)
`L.sort()`	Sort `L` in place (does not make a copy)

Slicing

Slicing allows you to extract parts of a list. The general form is:

L [s : e : i ]

Here, s is the starting index, e is the index just beyond the end of the slice, and i is the increment.

Here are some examples using the list

L = [ 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h' ]

L[2] is 'c'
L[-1] is 'h'
L[2:4] is [ 'c', 'd' ]
L[2:] is [ 'c', 'd', 'e', 'f', 'g', 'h' ]
L[:4] is [ 'a', 'b', 'c', 'd' ]
L[2:8] is [ 'c', 'd', 'e', 'f', 'g', 'h' ]
L[2:8:2] is [ 'c', 'e', 'g' ]
L[8:2:-1] is [ 'h', 'g', 'f', 'e', 'd' ]
L[8:2:-2] is [ 'h', 'f', 'd' ]
L[2:8:-1] is [ ]
L[:] is [ 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h' ]

Warning

Be careful when you are assigning lists! Here’s a code segment that may surprise you when it is executed:


L = [ 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h' ]
X = L
print("Before, X is", X)
print("Before, L is", L)
X[2] = 'x'
print("After, X is", X)
print("After, L is", L)

This prints:


Before, X is ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
Before, L is ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
After, X is ['a', 'b', 'x', 'd', 'e', 'f', 'g', 'h']
After, L is ['a', 'b', 'x', 'd', 'e', 'f', 'g', 'h']

Notice that when you change the list X, you also change L. That’s because X is just another name for L.

If you want to make a copy of L, you must do it this way:


L = [ 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h' ]
X = L[:]
print("Before, X is", X)
print("Before, L is", L)
X[2] = 'x'
print("After, X is", X)
print("After, L is", L)

This prints:


Before, X is ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
Before, L is ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
After, X is ['a', 'b', 'x', 'd', 'e', 'f', 'g', 'h']
After, L is ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']

Here, while X is changed, L is not. So L[:] creates a copy of the list L, which is then given the name (assigned to the variable) X.

Matt Bishop
Office: 2209 Watershed Sciences
Phone: +1 (530) 752-8060
Email: mabishop@ucdavis.edu

MHI 289I, Programming in Health Informatics
Version of September 16, 2025 at 12:38PM

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