Does anyone know why Python's list.append method is not called list.push, given that there's already a list.pop that removes and returns the last element (indexed at -1) and list.append semantic is consistent with that usage?
12 Answers
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.
5 Comments
stevenjackson121
@poige
you're going to *read* code that uses the other one (...) which is more cognitive load Remembering "there's no push" only introduces cognitive load when you're writing code. Remembering "push is an exact synonym for append" introduces cognitive load whenever you read the one you see used less often. See stackoverflow.com/questions/3455488/… for more about why people think readability often trumps writeability
poige
Makes none of excuse/sense
Ed Randall
3455488 is 404 ... not sure whether that reflects more on s/o policies than that argument though...?
Karol Zlot
@EdRandall this question is archived here: web.archive.org/web/20170422054706/https://stackoverflow.com/…
Faelin
push and pop terminology has been used with lists and stacks since the late 1950s to refer to adding/removing elements at the higher-index ("top" / "end") of a sequential data structure. Guido's choice to use append was not simply because "pop hadn't been suggested yet." As for cognitive load, any programmer who wants to be able to use more than one language should be aware of the concept of "push/pop", and should also know how "directionality" of a linear data structure effects the push and pop operations.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.
http://docs.python.org/tutorial/datastructures.html
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.
14 Comments
Uri
The tutorial seems to suggest that it simply pushes and pops from the end: "The list methods make it very easy to use a list as a stack, where the last element added is the first element retrieved (“last-in, first-out”). To add an item to the top of the stack, use append(). To retrieve an item from the top of the stack, use pop() without an explicit index. "
Kip
"pushing" in no way means adding to the front. every implementation of a stack that has ever been written by a sane person "pushes" onto the top (end) of the stack, not the bottom (start) of the stack
Kip
correction: every *array-based implementation. a linked-list implementation would push to the head.
Kobi
javascript
push adds to the end.
fortran
No, having into account
list.pop semantics, list.append pushes elements into the list, when viewed as a stack. |
Because it appends an element to a list? Push is usually used when referring to stacks.
3 Comments
Jason Baker
A list can be a stack though. :-)
Mark E. Haase
@JasonBaker You can implement a stack using a list, but that doesn't mean list==stack. You could also implement a stack using a queue, if you really wanted. (It would be horribly inefficient, but it's possible!)
JesperE
The confusion really comes from the fact that a stack doesn't have a "beginning" or "end" like a list, but rather a "top" and a "bottom". Adding to the stack implies placing an element on the top and "pushing" down. "Pushing" at the front makes no sense (at least not linguistically). And just to make things even more confusing, C++ uses "push_front" and "push_back".
Because "append" intuitively means "add at the end of the list". If it was called "push", then it would be unclear whether we're adding stuff at the tail or at head of the list.
1 Comment
jamesdlin
This doesn't make sense since there's a
pop operation. Since push and pop are typically stack operations and go together, it should be expected that they operate on the same end of the list.Not an official answer by any means (just a guess based on using the language), but Python allows you to use lists as stacks (e.g., section 5.1.1 of the tutorial). However, a list is still first of all a list, so the operations that are common to both use list terms (i.e., append) rather than stack terms (i.e., push). Since a pop operation isn't that common in lists (though 'removeLast' could have been used), they defined a pop() but not a push().
Comments
FYI, it's not terribly difficult to make a list that has a push method:
>>> class StackList(list):
... def push(self, item):
... self.append(item)
...
>>> x = StackList([1,2,3])
>>> x
[1, 2, 3]
>>> x.push(4)
>>> x
[1, 2, 3, 4]
A stack is a somewhat abstract datatype. The idea of "pushing" and "popping" are largely independent of how the stack is actually implemented. For example, you could theoretically implement a stack like this (although I don't know why you would):
l = [1,2,3]
l.insert(0, 1)
l.pop(0)
...and I haven't gotten into using linked lists to implement a stack.
2 Comments
kriss
No need to make it complicated:
class StackList(type([])): push = StackList.append will be enough to define your class.
NoBlockhit
Uhm, this should be insert(item, 0) instead of append(item) !!!!
Ok, personal opinion here, but Append and Prepend imply precise positions in a set.
Push and Pop are really concepts that can be applied to either end of a set... Just as long as you're consistent... For some reason, to me, Push() seems like it should apply to the front of a set...
1 Comment
MarkHu
Since you brought it up, if arrays have an .append() function, then why is there no corresponding.prepend() function? I can learn to use .insert(0,val) to prepend, but am then embarrassed by the lack of a corresponding .delete(pos,val) function. ref: docs.python.org/2/library/array.html
From PEP 20 -- The Zen of Python:
There should be one-- and preferably only one --obvious way to do it.
Having both list.append and list.push would be two ways of doing the same thing -- and list.append came first.
Comments
Push is a defined stack behaviour; if you pushed A on to stack (B,C,D) you would get (A,B,C,D).
If you used python append, the resulting dataset would look like (B,C,D,A)
Edit: Wow, holy pedantry.
I would assume that it would be clear from my example which part of the list is the top, and which part is the bottom. Assuming that most of us here read from left to right, the first element of any list is always going to be on the left.
3 Comments
fortran
That's not true, pop removes from the end of the list, not from the front.
Kip
read the page you link to. push is defined as pushing onto the top of the stack. which end is the "top" depends on the implementation. in an array-based stack, push would push onto the end of the array. in a linked-list-based stack, push would push to the beginning.
psmears
It's not really pedantry when the Python function
pop() exists and already removes an element from the end of the list. So the convention is already set, and it's that way round.Probably because the original version of Python (CPython) was written in C, not C++.
The idea that a list is formed by pushing things onto the back of something is probably not as well-known as the thought of appending them.
4 Comments
Jason Baker
The second part is a good answer. But what does that have to do with being implemented in C/C++?
unwind
@Jason: In C++'s STL, push_back() is how you append to a list. I was trying to convey the meta-idea that the idea that lists are formed by pushing is perhaps more likely to pop up if you're working in C++. Make any sense?
Andrew Dalke
If you have a list type implemented as a contiguous array (a vector in C++, a list in Python, an array in Perl) then it makes sense to have "push" put the new element at the end. You'll please note that perl 4 supposed "push" and "pop" as functions on arrays exactly like Python's append/pop and C++'s push_back/pop_back, and well before STL was formally proposed to C++. So it has nothing to do with C++'s STL creating a new understanding of things.
Peter
One of the things I miss learning Python from a Perl background is the ability to use built-in push(), pop(), shift(), and unshift() operations to add/remove elements to/from either end of the same array. Even though I can easily wrap a Python list in a "Stackish" class or a "Queueish" class, it doesn't look so easy (or efficient) to do both at once.
Push and Pop make sense in terms of the metaphor of a stack of plates or trays in a cafeteria or buffet, specifically the ones in type of holder that has a spring underneath so the top plate is (more or less... in theory) in the same place no matter how many plates are under it.
If you remove a tray, the weight on the spring is a little less and the stack "pops" up a little, if you put the plate back, it "push"es the stack down. So if you think about the list as a stack and the last element as being on top, then you shouldn't have much confusion.
Comments
I am a python newbie, for myself, I use this:
def push(one, array):
array.append(one)
def pop(array):
if len(array) > 0:
one = array[len(array)-1]
del array[len(array)-1]
return one
else:
return None
def pop_first(array):
if len(array) > 0:
one = array[0]
del array[0]
return one
else:
return None
pop_first() is to add from beginning of the list (the opposite of pop())
I don't use append just to make my code more readable (from my point of view), since I also code in JS, flutter, and PHP, function name similarities is important.
popcan pop items out from anywhere in a list.appendcannot "push" something into the middle of a list.