Python list push pop

Python pop and push option in list python

Guido wrote: You can also see he discusses the idea of if push/pop/put/pull should be at element [0] or after element [-1] where he posts a reference to Icon’s list: In other words, for stacks implemented directly as Python lists, which already supports fast append(), and del list[-1], it makes sense that list.pop() work by default on the last element. Or you can use: This way you can categories the URL’s based on domain and also can use for unique domain.

Why do python lists have pop() but not push()

Because «append» existed long before «pop» was thought of. Python 0.9.1 supported list.append in early 1991. By comparison, here’s part of a discussion on comp.lang.python about adding pop in 1997. Guido wrote:

To implement a stack, one would need to add a list.pop() primitive (and no, I’m not against this particular one on the basis of any principle). list.push() could be added for symmetry with list.pop() but I’m not a big fan of multiple names for the same operation — sooner or later you’re going to read code that uses the other one, so you need to learn both, which is more cognitive load.

You can also see he discusses the idea of if push/pop/put/pull should be at element [0] or after element [-1] where he posts a reference to Icon’s list:

I stil think that all this is best left out of the list object implementation — if you need a stack, or a queue, with particular semantics, write a little class that uses a lists

In other words, for stacks implemented directly as Python lists, which already supports fast append(), and del list[-1], it makes sense that list.pop() work by default on the last element. Even if other languages do it differently.

Implicit here is that most people need to append to a list, but many fewer have occasion to treat lists as stacks, which is why list.append came in so much earlier.

Because it appends; it doesn’t push. «Appending» adds to the end of a list, «pushing» adds to the front.

Think of a queue vs. a stack.

Edit: To reword my second sentence more exactly, «Appending» very clearly implies adding something to the end of a list, regardless of the underlying implementation. Where a new element gets added when it’s «pushed» is less clear. Pushing onto a stack is putting something on «top,» but where it actually goes in the underlying data structure completely depends on implementation. On the other hand, pushing onto a queue implies adding it to the end.

Because it appends an element to a list? Push is usually used when referring to stacks.

Python-push and pop from list at the same time, Please, include your code in your post directly instead of a screenshot. Also, you don’t necessarily have to use pop(which also removes the

How to pop item from list and push it back to list based on condition using python

Your algorithm is almost correct, but not the implementation:

>>> L = [1,2,3] >>> L.pop() 3 >>> L.append(3) >>> L [1, 2, 3] 

That’s why your program loops forever: if the domain is the same as the previous domain, you just append then pop then append, then. What you need is not a stack, it’s a round robin:

>>> L.pop() 3 >>> L.insert(0, 3) >>> L [3, 1, 2] 

Let’s take a shuffled list of permutations of «abcd»:

>>> L = [('b', 'c', 'd', 'a'), ('d', 'c', 'b', 'a'), ('a', 'c', 'd', 'b'), ('c', 'd', 'a', 'b'), ('b', 'd', 'a', 'c'), ('b', 'a', 'd', 'c'), ('b', 'c', 'a', 'd'), ('a', 'b', 'd', 'c'), ('d', 'a', 'b', 'c'), ('a', 'b', 'c', 'd'), ('d', 'c', 'a', 'b'), ('a', 'd', 'c', 'b'), ('d', 'a', 'c', 'b'), ('c', 'd', 'b', 'a'), ('d', 'b', 'c', 'a'), ('d', 'b', 'a', 'c'), ('a', 'd', 'b', 'c'), ('b', 'd', 'c', 'a'), ('c', 'b', 'd', 'a'), ('c', 'a', 'b', 'd'), ('b', 'a', 'c', 'd')] 

The first letter is the domain . Here’s a slightly modified version of your code:

>>> prev = None >>> while L: . e = L.pop() . if L and e[0] == prev: . L.insert(0, e) . else: . print(e) . prev = e[0] ('b', 'a', 'c', 'd') ('c', 'a', 'b', 'd') ('b', 'd', 'c', 'a') ('a', 'd', 'b', 'c') ('d', 'b', 'a', 'c') ('c', 'd', 'b', 'a') ('d', 'a', 'c', 'b') ('a', 'd', 'c', 'b') ('d', 'c', 'a', 'b') ('a', 'b', 'c', 'd') ('d', 'a', 'b', 'c') ('a', 'b', 'd', 'c') ('b', 'c', 'a', 'd') ('c', 'd', 'a', 'b') ('a', 'c', 'd', 'b') ('d', 'c', 'b', 'a') ('b', 'c', 'd', 'a') ('c', 'b', 'd', 'a') ('d', 'b', 'c', 'a') ('b', 'a', 'd', 'c') ('b', 'd', 'a', 'c') 

The modification is: if L and , because if the last element of the list domain is prev , then you’ll loop forever with your one element list: pop, same as prev, insert, pop, . (as with pop/append)

Here’s another option: create a dict domain -> list of urls :

>>> d = <> >>> for e in L: . d.setdefault(e[0], []).append(e) >>> d

Now, take an element of every domain and clear the dict, then loop until the dict is empty:

>>> while d: . for k, vs in d.items(): . e = vs.pop() . print (e) . d = # clear the dict . ('b', 'a', 'c', 'd') ('d', 'b', 'a', 'c') ('a', 'd', 'b', 'c') ('c', 'a', 'b', 'd') ('b', 'd', 'c', 'a') ('d', 'b', 'c', 'a') ('a', 'd', 'c', 'b') ('c', 'b', 'd', 'a') ('b', 'c', 'a', 'd') ('d', 'a', 'c', 'b') ('a', 'b', 'c', 'd') ('c', 'd', 'b', 'a') ('b', 'a', 'd', 'c') ('d', 'c', 'a', 'b') ('a', 'b', 'd', 'c') ('c', 'd', 'a', 'b') ('b', 'd', 'a', 'c') ('d', 'a', 'b', 'c') ('a', 'c', 'd', 'b') ('b', 'c', 'd', 'a') ('d', 'c', 'b', 'a') 

The output is more uniform.

Check the following code snippet,

urls = ['http://domain1.com','http://domain1.com/page1','http://domain2.com'] crawl_for_urls = <> for url in urls: domain = base_url(url) if domain not in crowl_for_urls: crawl_for_urls.update() crawl(url) 

crawl() will be called only for unique domain.

urls = ['http://domain1.com','http://domain1.com/page1','http://domain2.com'] crawl_for_urls = <> for url in urls: domain = base_url(url) if domain not in crowl_for_urls: crawl_for_urls.update() crawl(url) else: crawl_for_urls.get(domain, []).append(url) 

This way you can categories the URL’s based on domain and also can use crawl() for unique domain.

How to pop item from list and push it back to list based on condition, Loop through all items in the list and compare current element base url with previously processed element base url. If base urls are different

What is the point of using the pop() on list? [duplicate]

They did implement push, but they split the functionality into list.insert() and list.append() instead.

list.append() is the equivalent of pushing a value onto the end. list.insert() is the inverse of list.pop() with an index; inserting the given value at the given index.

The list.pop() method is an alternative to del listobject[index] in that it returns the value at the index you are removing.

Note that Python lists are not limited to being used as a stack or queue, and list.pop() is a later addition to the list type to make removing-and-returning more efficient.

What about append() ? That’s the equivalent to push .

The whole purpose is a quick way to use a list as a stack when convenient. It can also be used as a queue with the combination of methods pop(0) and append() . Although for this specific cases the best choice is deque from collections.

Pop is annoying to do otherwise:

popped = my_list[-1] my_list = my_list[:-1] 

The second part of your question is already answered by others — it’s just called append instead.

Also see Why is Python’s «append» not «push»?

Wand push() and pop() in Python, push() function is used to grow context stack and pop() is another function and used to restore stack to previous push. Syntax :

Why list append and pop(0) faster than incrementing and decrementing front rear counters in queue?

Python is an interpreted language. It translates function execution into bytecodes for its «virtual machine».

Your enqueue function translates to the following operations:

>>> import dis #Python disassembler from std lib >>> dis.dis(enqueue) 3 0 LOAD_GLOBAL 0 (rear) 2 LOAD_CONST 1 (1) 4 BINARY_ADD 6 LOAD_GLOBAL 1 (size) 8 BINARY_MODULO 10 LOAD_GLOBAL 2 (front) 12 COMPARE_OP 2 (==) 14 POP_JUMP_IF_FALSE 28 4 16 LOAD_GLOBAL 3 (print) 18 LOAD_CONST 2 ("Queue is full. Can't add new item.") 20 CALL_FUNCTION 1 22 POP_TOP 5 24 LOAD_CONST 1 (1) 26 RETURN_VALUE 7 >> 28 LOAD_FAST 0 (x) 30 LOAD_GLOBAL 4 (L) 32 LOAD_GLOBAL 0 (rear) 34 STORE_SUBSCR 8 36 LOAD_GLOBAL 0 (rear) 38 LOAD_CONST 1 (1) 40 BINARY_ADD 42 LOAD_GLOBAL 1 (size) 44 BINARY_MODULO 46 STORE_GLOBAL 0 (rear) 9 48 LOAD_CONST 3 (0) 50 RETURN_VALUE 52 LOAD_CONST 0 (None) 54 RETURN_VALUE 

whereas append is a builtin operation, highly optimized and coded in C (for CPython).

I think this explains at least one reason why your code is much slower.

Optimisation is a difficult business, efficiency depends on many factors and there can be surprises at all levels so it is better to resist to premature optimization: only start looking at those details if you have a problem.

For comparison, here is the same disassembling for a function calling builtin append on a list:

>>> import dis >>> def append(l, x): . l.append(x) . >>> dis.dis(append) 2 0 LOAD_FAST 0 (l) 2 LOAD_METHOD 0 (append) 4 LOAD_FAST 1 (x) 6 CALL_METHOD 1 8 POP_TOP 10 LOAD_CONST 0 (None) 12 RETURN_VALUE >>> 

In fact it is just doing LOAD_METHOD and CALL_METHOD .

My pop( ) function does not delete the top element of a list, but rather, def push ; arr,a · if ; len · def ; isempty · arr ; if len

Источник

Списки (list). Функции и методы списков

Сегодня я расскажу о таком типе данных, как списки, операциях над ними и методах, о генераторах списков и о применении списков.

Что такое списки?

Списки в Python — упорядоченные изменяемые коллекции объектов произвольных типов (почти как массив, но типы могут отличаться).

Чтобы использовать списки, их нужно создать. Создать список можно несколькими способами. Например, можно обработать любой итерируемый объект (например, строку) встроенной функцией list:

Список можно создать и при помощи литерала:

Как видно из примера, список может содержать любое количество любых объектов (в том числе и вложенные списки), или не содержать ничего.

И еще один способ создать список — это генераторы списков. Генератор списков — способ построить новый список, применяя выражение к каждому элементу последовательности. Генераторы списков очень похожи на цикл for.

Возможна и более сложная конструкция генератора списков:

Но в сложных случаях лучше пользоваться обычным циклом for для генерации списков.

Функции и методы списков

Создать создали, теперь нужно со списком что-то делать. Для списков доступны основные встроенные функции, а также методы списков.

Таблица «методы списков»

Нужно отметить, что методы списков, в отличие от строковых методов, изменяют сам список, а потому результат выполнения не нужно записывать в эту переменную.

   И, напоследок, примеры работы со списками:

Изредка, для увеличения производительности, списки заменяют гораздо менее гибкими массивами (хотя в таких случаях обычно используют сторонние библиотеки, например NumPy).

Для вставки кода на Python в комментарий заключайте его в теги