>>> li =['a','b','mpilgrim','z','example']>>> li
['a','b','mpilgrim','z','example']>>> li.append("new")>>> li
['a','b','mpilgrim','z','example','new']>>> li.append(["new",2])>>> li
['a','b','mpilgrim','z','example','new',['new',2]]>>> li.insert(2,"new")>>> li
['a','b','new','mpilgrim','z','example','new',['new',2]]>>> li.extend(["two","elements"])>>> li
['a','b','new','mpilgrim','z','example','new',['new',2],'two','elements']
def append_one(a_list, element):
a_list.append(element)def extend_one(a_list, element):"""creating a new list is semantically the most direct
way to create an iterable to give to extend"""
a_list.extend([element])import timeit
What is the difference between the list methods append and extend?
append adds its argument as a single element to the end of a list. The length of the list itself will increase by one.
extend iterates over its argument adding each element to the list, extending the list. The length of the list will increase by however many elements were in the iterable argument.
append
The list.append method appends an object to the end of the list.
my_list.append(object)
Whatever the object is, whether a number, a string, another list, or something else, it gets added onto the end of my_list as a single entry on the list.
So keep in mind that a list is an object. If you append another list onto a list, the first list will be a single object at the end of the list (which may not be what you want):
>>> another_list = [1, 2, 3]
>>> my_list.append(another_list)
>>> my_list
['foo', 'bar', 'baz', [1, 2, 3]]
#^^^^^^^^^--- single item at the end of the list.
extend
The list.extend method extends a list by appending elements from an iterable:
my_list.extend(iterable)
So with extend, each element of the iterable gets appended onto the list. For example:
Keep in mind that a string is an iterable, so if you extend a list with a string, you’ll append each character as you iterate over the string (which may not be what you want):
Both + and += operators are defined for list. They are semantically similar to extend.
my_list + another_list creates a third list in memory, so you can return the result of it, but it requires that the second iterable be a list.
my_list += another_list modifies the list in-place (it is the in-place operator, and lists are mutable objects, as we’ve seen) so it does not create a new list. It also works like extend, in that the second iterable can be any kind of iterable.
Don’t get confused – my_list = my_list + another_list is not equivalent to += – it gives you a brand new list assigned to my_list.
Iterating through the multiple calls to append adds to the complexity, making it equivalent to that of extend, and since extend’s iteration is implemented in C, it will always be faster if you intend to append successive items from an iterable onto a list.
Performance
You may wonder what is more performant, since append can be used to achieve the same outcome as extend. The following functions do the same thing:
def append(alist, iterable):
for item in iterable:
alist.append(item)
def extend(alist, iterable):
alist.extend(iterable)
Perfect answer, I just miss the timing of comparing adding only one element
Do the semantically correct thing. If you want to append all elements in an iterable, use extend. If you’re just adding one element, use append.
Ok, so let’s create an experiment to see how this works out in time:
def append_one(a_list, element):
a_list.append(element)
def extend_one(a_list, element):
"""creating a new list is semantically the most direct
way to create an iterable to give to extend"""
a_list.extend([element])
import timeit
And we see that going out of our way to create an iterable just to use extend is a (minor) waste of time:
We learn from this that there’s nothing gained from using extend when we have only one element to append.
Also, these timings are not that important. I am just showing them to make the point that, in Python, doing the semantically correct thing is doing things the Right Way™.
It’s conceivable that you might test timings on two comparable operations and get an ambiguous or inverse result. Just focus on doing the semantically correct thing.
Conclusion
We see that extend is semantically clearer, and that it can run much faster than append, when you intend to append each element in an iterable to a list.
If you only have a single element (not in an iterable) to add to the list, use append.
回答 3
append追加一个元素。extend追加元素列表。
请注意,如果您传递要追加的列表,它仍会添加一个元素:
>>> a =[1,2,3]>>> a.append([4,5,6])>>> a
[1,2,3,[4,5,6]]
With append you can append a single element that will extend the list:
>>> a = [1,2]
>>> a.append(3)
>>> a
[1,2,3]
If you want to extend more than one element you should use extend, because you can only append one elment or one list of element:
>>> a.append([4,5])
>>> a
>>> [1,2,3,[4,5]]
So that you get a nested list
Instead with extend, you can extend a single element like this
>>> a = [1,2]
>>> a.extend([3])
>>> a
[1,2,3]
Or, differently, from append, extend more elements in one time without nesting the list into the original one (that’s the reason of the name extend)
>>> a.extend([4,5,6])
>>> a
[1,2,3,4,5,6]
Adding one element with both methods
Both append and extend can add one element to the end of the list, though append is simpler.
append 1 element
>>> x = [1,2]
>>> x.append(3)
>>> x
[1,2,3]
extend one element
>>> x = [1,2]
>>> x.extend([3])
>>> x
[1,2,3]
Adding more elements… with different results
If you use append for more than one element, you have to pass a list of elements as arguments and you will obtain a NESTED list!
>>> x = [1,2]
>>> x.append([3,4])
>>> x
[1,2,[3,4]]
With extend, instead, you pass a list as an argument, but you will obtain a list with the new element that is not nested in the old one.
>>> z = [1,2]
>>> z.extend([3,4])
>>> z
[1,2,3,4]
So, with more elements, you will use extend to get a list with more items. However, appending a list will not add more elements to the list, but one element that is a nested list as you can clearly see in the output of the code.
The append() method adds a single item to the end of the list.
x = [1, 2, 3]
x.append([4, 5])
x.append('abc')
print(x)
# gives you
[1, 2, 3, [4, 5], 'abc']
The extend() method takes one argument, a list, and appends each of the items of the argument to the original list. (Lists are implemented as classes. “Creating” a list is really instantiating a class. As such, a list has methods that operate on it.)
x = [1, 2, 3]
x.extend([4, 5])
x.extend('abc')
print(x)
# gives you
[1, 2, 3, 4, 5, 'a', 'b', 'c']
Similarly += for in place behavior, but with slight differences from append & extend. One of the biggest differences of += from append and extend is when it is used in function scopes, see this blog post.
回答 8
append(object) -通过将对象添加到列表来更新列表。
x =[20]# List passed to the append(object) method is treated as a single object.
x.append([21,22,23])# Hence the resultant list length will be 2print(x)-->[20,[21,22,23]]
extend(list) -本质上是串联两个列表。
x =[20]# The parameter passed to extend(list) method is treated as a list.# Eventually it is two lists being concatenated.
x.extend([21,22,23])# Here the resultant list's length is 4print(x)[20,21,22,23]
append(object) – Updates the list by adding an object to the list.
x = [20]
# List passed to the append(object) method is treated as a single object.
x.append([21, 22, 23])
# Hence the resultant list length will be 2
print(x)
--> [20, [21, 22, 23]]
extend(list) – Essentially concatenates two lists.
x = [20]
# The parameter passed to extend(list) method is treated as a list.
# Eventually it is two lists being concatenated.
x.extend([21, 22, 23])
# Here the resultant list's length is 4
print(x)
[20, 21, 22, 23]
An interesting point that has been hinted, but not explained, is that extend is faster than append. For any loop that has append inside should be considered to be replaced by list.extend(processed_elements).
Bear in mind that apprending new elements might result in the realloaction of the whole list to a better location in memory. If this is done several times because we are appending 1 element at a time, overall performance suffers. In this sense, list.extend is analogous to “”.join(stringlist).
Append adds the entire data at once. The whole data will be added to the newly created index. On the other hand, extend, as it name suggests, extends the current array.
lis =[1,2,3]# 'extend' is equivalent to this
lis = lis + list(iterable)# 'append' simply appends its argument as the last element to the list# as long as the argument is a valid Python object
list.append(object)
An English dictionary defines the words append and extend as:
append: add (something) to the end of a written document. extend: make larger. Enlarge or expand
With that knowledge, now let’s understand
1) The difference between append and extend
append:
Appends any Python object as-is to the end of the list (i.e. as a the last element in the list).
The resulting list may be nested and contain heterogeneous elements (i.e. list, string, tuple, dictionary, set, etc.)
extend:
Accepts any iterable as its argument and makes the list larger.
The resulting list is always one-dimensional list (i.e. no nesting) and it may contain heterogeneous elements in it (e.g. characters, integers, float) as a result of applying list(iterable).
2) Similarity between append and extend
Both take exactly one argument.
Both modify the list in-place.
As a result, both returns None.
Example
lis = [1, 2, 3]
# 'extend' is equivalent to this
lis = lis + list(iterable)
# 'append' simply appends its argument as the last element to the list
# as long as the argument is a valid Python object
list.append(object)
I hope I can make a useful supplement to this question. If your list stores a specific type object, for example Info, here is a situation that extend method is not suitable: In a for loop and and generating an Info object every time and using extend to store it into your list, it will fail. The exception is like below:
TypeError: ‘Info’ object is not iterable
But if you use the append method, the result is OK. Because every time using the extend method, it will always treat it as a list or any other collection type, iterate it, and place it after the previous list. A specific object can not be iterated, obviously.
def append_o(a_list, element):
a_list.append(element)print('append:', end =' ')for item in a_list:print(item, end =',')print()def extend_o(a_list, element):
a_list.extend(element)print('extend:', end =' ')for item in a_list:print(item, end =',')print()
append_o(['ab'],'cd')
extend_o(['ab'],'cd')
append_o(['ab'],['cd','ef'])
extend_o(['ab'],['cd','ef'])
append_o(['ab'],['cd'])
extend_o(['ab'],['cd'])
append “extends” the list (in place) by only one item, the single object passed (as argument).
extend “extends” the list (in place) by as many items as the object passed (as argument) contains.
This may be slightly confusing for str objects.
If you pass a string as argument:
append will add a single string item at the end but
extend will add as many “single” ‘str’ items as the length of that string.
If you pass a list of strings as argument:
append will still add a single ‘list’ item at the end and
extend will add as many ‘list’ items as the length of the passed list.
def append_o(a_list, element):
a_list.append(element)
print('append:', end = ' ')
for item in a_list:
print(item, end = ',')
print()
def extend_o(a_list, element):
a_list.extend(element)
print('extend:', end = ' ')
for item in a_list:
print(item, end = ',')
print()
append_o(['ab'],'cd')
extend_o(['ab'],'cd')
append_o(['ab'],['cd', 'ef'])
extend_o(['ab'],['cd', 'ef'])
append_o(['ab'],['cd'])
extend_o(['ab'],['cd'])
Append and extend are one of the extensibility mechanisms in python.
Append: Adds an element to the end of the list.
my_list = [1,2,3,4]
To add a new element to the list, we can use append method in the following way.
my_list.append(5)
The default location that the new element will be added is always in the (length+1) position.
Insert: The insert method was used to overcome the limitations of append. With insert, we can explicitly define the exact position we want our new element to be inserted at.
Method descriptor of insert(index, object). It takes two arguments, first being the index we want to insert our element and second the element itself.
Extend: This is very useful when we want to join two or more lists into a single list. Without extend, if we want to join two lists, the resulting object will contain a list of lists.
a = [1,2]
b = [3]
a.append(b)
print (a)
[1,2,[3]]
If we try to access the element at pos 2, we get a list ([3]), instead of the element. To join two lists, we’ll have to use append.
a = [1,2]
b = [3]
a.extend(b)
print (a)
[1,2,3]
To join multiple lists
a = [1]
b = [2]
c = [3]
a.extend(b+c)
print (a)
[1,2,3]