Strange behavior of lists in python [duplicate]

by

When you multiply a list, it copies a reference to the list, it doesn’t create a copy of the list. As lists are mutable, when you change it, it is changed for all the references to it.

In ASCII-art terms:

a1 --- [list1, list2] --- list1 = [0, 0]
                      --- list2 = [0, 0]

a2 --- [list3, list3] --- list3 = [0, 0]

You can clearly see that changing list3 will affect both positions.

If you want to create variable-length lists without copying references, you should instead do something like this:

>>> a2 = [[0]*2 for _ in range(2)]
>>> a2[0][0] = 1
>>> a2
[[1, 0], [0, 0]]

Here we are using a list comprehension in order to create new elements each time, rather than copying references to old elements. In more complex situations, where you have existing mutable objects you want to repeat, you might want to use the copy module.

Note the [0]*2 operation is still OK, as ints in Python are immutable, and can’t change, so it doesn’t matter if you have references to the same object.

More Related Contents:

  1. Print cubes of the numbers 1 through 10 only if the cube is evenly divisible by four
  2. How can I calculate a list modulo a numer in python?
  3. How to read video stream as input in OpenCV python?
  4. How to do for loop through all items of a list except the first [duplicate]
  5. How to select a drop-down menu value with Selenium using Python?
  6. How do I get a list of locally installed Python modules?
  7. How do I create a list of lambdas (in a list comprehension/for loop)?
  8. Calling a python script with input within a python script using subprocess
  9. Add text to Existing PDF using Python
  10. Django Passing Custom Form Parameters to Formset
  11. Python Socket Receive Large Amount of Data
  12. How do I count occurrence of unique values inside a list
  13. Spark Dataframe distinguish columns with duplicated name
  14. Python Decimals format
  15. Getting rid of console output when freezing Python programs using Pyinstaller
  16. How to sort and remove duplicates from Python list? [duplicate]
  17. Case insensitive dictionary
  18. Python OpenCV load image from byte string
  19. Sorting a dictionary with lists as values, according to an element from the list
  20. How to check if the current time is in range in python?
  21. How to make Django serve static files with Gunicorn?
  22. Merge several Python dictionaries
  23. Make Anaconda’s tkinter aware of system fonts or install new fonts for Anaconda
  24. Pandas custom function to find whether it is the 1st, 2nd etc Monday, Tuesday, etc – all suggestions welcome
  25. stop tkinter window from freezing while program is sleeping
  26. Resize fields in Django Admin
  27. How do I get the user agent with Flask?
  28. Creating a list in Python with multiple copies of a given object in a single line
  29. How to document Python packages using Sphinx
  30. Pytorch ValueError: optimizer got an empty parameter list

Leave a Comment