9.2. Methods, Operators and Functions on Lists

9.2. Methods, Operators and Functions on Lists
	Chapter 9. Collections

Table 9.1 remembers the action of builtin functions and operators on list objects and Table 9.2 summarizes all methods of list objects.

Table 9.1. Sequence types: Operators and Functions

Operator/Function Action Action on Numbers

[ ... ], ( ... ), " ... " creation

s + t concatenation addition

s * n repetition ^[a] multiplication

s[i] indication

s[i:k] slice

x in s membership

x not in s

for a in s iteration

len(s) length

min(s) return smallest element

max(s) return greatest element

s[i] = x index assignment

s[i:k] = t slice assignment

del s[i] deletion

	Important
	^[a]shallow copy (see Section 10.2)

Table 9.2. List methods

Method	Operation
`list(s)`	converts any sequence object to a list
`s.append(x)`	append a new element
`s.extend(t)`	concatenation^[a]
`s.count(x)`	count occurrences of `x`
`s.index(x)`	find smallest position where `x` occurs in s
`s.insert(i,x)`	insert `x` at position `i`
`s.pop([i])`	removes i-th element and return it
`s.remove(x)`	remove element
`s.reverse()`^[b]	reverse
`s.sort([cmp])`^[b]	sort according to the `cmp` function
^[a]equal to the `+` operator but inplace ^[b]in place operation

Important

	Important
It is important to know whether a function or method, that is applied to a mutable objects, modifies this object internally or whether it returns a new object containing these modifications. Look at the following example that shows two possibilities to concatenate lists. The `+` operator creates a new list whereas the method `extend` adds one list to the other: >>> l1 = [ 'EcoRI', 'BamHI' ] >>> l2 = [ 'HindIII' ] >>> l1 ['EcoRI', 'BamHI'] >>> l2 ['HindIII'] >>> l1 + l2 ['EcoRI', 'BamHI', 'HindIII'] >>> l1 ['EcoRI', 'BamHI'] >>> l2 ['HindIII'] >>> l1.extend(l2) >>> l1 ['EcoRI', 'BamHI', 'HindIII'] >>> l2 ['HindIII']

It is important to know whether a function or method, that is applied to a mutable objects, modifies this object internally or whether it returns a new object containing these modifications. Look at the following example that shows two possibilities to concatenate lists. The + operator creates a new list whereas the method extend adds one list to the other:

>>> l1 = [ 'EcoRI', 'BamHI' ]
>>> l2 = [ 'HindIII' ]
>>> l1
['EcoRI', 'BamHI']
>>> l2
['HindIII']
>>> l1 + l2
['EcoRI', 'BamHI', 'HindIII']
>>> l1 
['EcoRI', 'BamHI']
>>> l2
['HindIII']
>>> l1.extend(l2)
>>> l1
['EcoRI', 'BamHI', 'HindIII']
>>> l2
['HindIII']


Chapter 9. Collections		9.3. Methods, Operators and Functions on Dictionaries