Python list update in place

Find centralized, trusted content and collaborate around the technologies you use most. Connect and share knowledge within a single location that is structured and easy to search. I am using Python's collections library to make a dictionary where the keys are integers and the values are lists.

I am using the defaultdict list command. I am trying, but unsuccessfully to edit the elements in these lists which are values. I thought list comprehension should work for this but I keep getting syntax errors. I attach what I have tried below:. Assuming you want to modify the list in place, you need to overwrite each element as integers are immutable:.

If you don't care about generating a new object i. The first case modifies the list in place. If other variables point to the list, the changes will be reflected:. In the other case, the list is replaced by a new, independent, one,.

All potential bindings are lost:. The way you are using the for loops doesn't affect the actual list, just the temporary variable that is iterating through the list. There are a few ways you can fix this. Instead of iterating through each element you can can count up to the length of the list and modify the list directly. You can also use the enumerate function to iterate through both a counter and list items. Finally, you can use list comprehension to generate a new list with desired differences in one line.

The usual way to poke values into an existing list in Python is to use enumerate which lets you iterate over both the indices and the values at once -- then use the indices to manipulate the list:. In the second example the list objects in A become the loop variable row -- and since you're only mutating them not assigning to them you don't need enumerate and the index. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. In-place modification of Python lists Ask Question. Asked 3 years, 5 months ago. Active 10 months ago. Viewed 26k times. Let's take another example: A.

Return Value from update This set update method returns None returns nothing. Previous Tutorial:. Share on:. Did you find this article helpful? Sorry about that. How can we improve it?

Leave this field blank. Python References. Thanks for the tip. How would you do it instead? Make a copy and iterate though it: for num in nums[:]: , then you're safe to modify the original list. I think I mention this above but it should have been explicitly stated , but the original problem states to "move all 0's to the end of it while maintaining the relative order of the non-zero elements and doing so in-place without making a copy of the array".

The you'd better do for i in range len nums and access the array by indices. Show 1 more comment. Active Oldest Votes. No How does it work? Improve this answer. Add a comment. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password.