Manipulating Lists

There are a number of ways to manipulate lists in Python. The following table describes them:

Function or Operator	Description
+	Join two lists together
append	Add a value to the end of a list
del	Delete a value from a list
remove	Remove a specified element from a list
reverse	Reverse the order of a list
sort	Sort a list in numeric or alphabetical order
len	Return the number of values in a list
index	Return the index of an item in a list
pop	Remove an item from a list and return it

You can use + to join two lists together:

idlist = [1000, 2000] + [3000, 4000]
# idlist now contains [1000, 2000, 3000, 4000]

You can use append to add a value to the end of a list:

idlist = [1000, 2000, 3000]
idlist.append(4000)   # idlist is now [1000, 2000, 3000, 4000]

To delete a value from a list, use del, which expects the index of the list value it is deleting:

idlist = [1000, 2000, 3000, 4000]
del idlist[1]
# idlist is now [1000, 3000, 4000] - recall that index 1 refers to the
# second element of the list, not the first

If you know what element you want to delete, but do not know where in the list it is, use remove:

idlist = [1000, 2000, 3000, 4000]
idlist.remove(2000);   # idlist is now [1000, 3000, 4000]

To reverse the order of a list, use reverse:

idlist = [1000, 2000, 3000, 4000]
idlist.reverse()  # idlist is now [4000, 3000, 2000, 1000]

sort sorts a list of numbers in numeric order, or a list of strings in alphabetical order:

idlist = [3000, 1000, 4000, 2000]
idlist.sort()  # idlist is now [1000, 2000, 3000, 4000]
namelist = ['Doe,John', 'Smith,Mary', 'Jones,Charlie', 'Public,John']
namelist.sort()
# namelist is now ['Doe,John', 'Jones,Charlie', 'Public,John', 'Smith,Mary']


	sort does not do anything if the list is a mixture of numbers and strings.

len returns the number of values in a list:

idlist = [1000, 2000, 3000, 4000]
idcount = len(idlist)   # idcount is now 4

index locates an item in a list, and returns its index:

patientinfo = ['JohnDoe', 42446, 'Admitted', 1000]
statusindex = patientinfo.index('Admitted')

In this example, index finds the first occurrence of the string Admitted in the list. Since this is the third element of the list, statusindex is assigned 2. (Recall that indexing always starts from zero.)

pop removes an item from a list, specified by index, and allows you to assign it to another variable:

idlist = [1000, 2000, 3000, 4000]
patientid = idlist.pop(1)
# idlist is now [1000, 3000, 4000], and patientid now contains 2000

If you do not specify an index, pop removes the last element of the list and assigns it:

idlist = [1000, 2000, 3000, 4000]
patientid = idlist.pop()
# idlist is now [1000, 2000, 3000], and patientid now contains 4000

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Manipulating Lists