Why is list.append() writing over data in this code

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I'm trying to append some lists to another list in a for loop, but it seems the append method is writing over the previous data of the list.

listFil1 = ["R1000", "D940", "L143", "D182", ...]

position1 = [0,0]

position1list = []

for i in range(0, len(listFil1)):
    #Get the move from each list
    move1 = listFil1[i]

    #evaluate position1
    if move1[0] == "R":
        position1[1] += int(move1[1:])
    elif move1[0] == "L":
        position1[1] -= int(move1[1:])
    elif move1[0] == "U":
        position1[0] += int(move1[1:])
    elif move1[0] == "D":
        position1[0] -= int(move1[1:])

    #Append each move, this is where the problem is
    position1list.append(position1)

If I'm printing position1 inside the for loop, I get the expect results for each iteration: * [0, 1000] * [-940, 1000] * [-940, 857] * [-1122, 857] * [-1122, -20] * [-1831, -20] * [-1831, -273] * [-1583, -273] * [-1583, -574] * [-1149, -574] * [-1149, 267] * [-434, 267] * [-434, 968] * ... * [-1966, -12739] * [-1966, -13333]

But when I print position1list at the end, outside of the for loop, I get the right length of data, but every item in the list is equal to the last position1: [[-1966, -13333], [-1966, -13333], [-1966, -13333], [-1966, -13333], [-1966, -13333], [-1966, -13333], [-1966, -13333], [-1966, -13333], [-1966, -13333], [-1966, -13333], [-1966, -13333],...]

Which part of the code is making append write over the whole list ? What can I change to get a list of all the positions at the end ?

You’re appending the same list reference, so they all have the same elements. You should make a shallow copy each time you append:

position1list.append(position1.copy())

Append vs Extend in Python Lists, Lists are one of the most useful data structures available in Python, or really In this article I'll be showing the differences between the append , extend , and insert list methods. Running this code will result in the following output: This method adds elements (notice its plural!) to a list by adding on all the insert(), O (n). Append: Adds its argument as a single element to the end of a list. The length of the list increases by one. syntax: # Adds an object (a number, a string or a # another list) at the end of my_list my_list.append(object)

You are using the same position1 object on all iterations and then simply appending multiple references to the same list inside position1list. You need to re-declare your list on each iteration to create a unique object.


A simple demonstration of your problem is the following:

>>> l = []
>>> a = [l, l]
>>> a
[[], []]
>>> l.append(1)
>>> a
[[1], [1]]

As you can see, because we created a list with l multiple times, appending to l changes the whole a list

Append in Python, You can see from the above code that while creating a list I have given The append() method in python adds a single item to the existing list. append() Parameters. The method takes a single argument. item - an item to be added at the end of the list; The item can be numbers, strings, dictionaries, another list, and so on.

This is happening because the variable position1 does not contain directly an array, it contains a reference to the array. That means that if you assign the array to another variable, and the modify one of the variables, both will be modified.

Example :

>>> a = [1,2]
>>> b = a
>>> a
[1, 2]
>>> b
[1, 2]
>>> b[0] = 0
>>> a
[0, 2]
>>> b
[0, 2]

So to deal with this, you should do a copy of your array :

>>> a = [1,2]
>>> b = a.copy()
>>> b[0] = 0
>>> b
[0, 2]
>>> a
[1, 2]

Python List Append VS Python List Extend – The Difference , It's because the . append() method adds the entire item to the end of the list. If the item is a sequence such as a list, dictionary, or tuple, the entire sequence will be added as a single item of the existing list. Definition and Usage. The append() method appends an element to the end of the list.

Python List Append – How to Add an Element to an Array, Explained , Python List Append – How to Add an Element to an Array, Explained with Examples If you want to learn how to use the append() method, then this article is for you. This element can be of any data type: To learn more about this, you can read my article: Python List Append VS Learn to code for free. The for/in constructs are very commonly used in Python code and work on data types other than list, so you should just memorize their syntax. You may have habits from other languages where you start manually iterating over a collection, where in Python you should just use for/in. You can also use for/in to work on a string.

Python Lists | Python Education, List literals are written within square brackets [ ]. Lists work similarly to strings -- use the len() function and square brackets The '+' works to append two lists, so [1, 2] + [3, 4] yields [1, 2, 3, The for/in constructs are very commonly used in Python code and work on data types other than list, so you should� Introduction This is a tutorial about writing code in Excel spreadsheets using Visual Basic for Applications (VBA). Excel is one of Microsoft’s most popular products. In 2016, the CEO of Microsoft said "Think about a world without Excel. That's just impossible for me.” Well, maybe the world can’t think without Excel. * In 1996, there were over 30 million users of Microsoft Excel (source

Python List append(), the list. In this tutorial, we will learn about the Python append() method in detail with the help of examples. The append() method adds an item to the end of the list. Run Code. Output Updated animals list: ['cat', 'dog', 'rabbit', 'guinea pig']� We used the range() function to generate an iterable sequence of numbers from 0 to 9. Then using the for loop, we iterated over that sequence and for each number in the sequence, we called the list’s append() function and passed the number to list.append() function, which adds the given item to the end of list in place.

Comments
  • Use position1list.append(position1[:]) to append a shallow copy at each stage. That will break the alaising that you are seeing.