How to remove items from a list while iterating? What did you expect? Then the pointer move to the new second previously third element. As long as you don't directly change the actual list, you're fine. Both interact with a global variable POST_QUEUE = [] Process 1 ( P1) adds items to POST_QUEUE every 60 seconds. (thanks to Wallacoloo), Like some dudes said, you could also use the extend-method of the list-object to just copy the one list to another, if this was your intention. Examples where the list is modified and not during while iterating over the elements of the list, list_not_modified_during_iteration.py (Changed item to i). This doesn't explain why in rogeriopvl's question the list contains two elements. Rather, you're only modifying the elements in lovely_numbers. for loop not removing all items from list,what is the solution? This deals with the pointer to a memory artifact. It might help to remember that lists only contain references (i.e. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. python - Modify a list while iterating - Stack Overflow But how much is it so, and is it safe to assume that behavior will hold true for future (and possibly past) versions of Python ? That way, no elements can be skipped since removing an item from the list will only affect the indexes of elements that were already handled. This is a well-documented behaviour in Python, that you aren't supposed to modify the list being iterated through. Traceback (most recent call last): and our :), Can I add or remove items to the list while the for loop is running. What is the most accurate way to map 6-bit VGA palette to 8-bit? Using robocopy on windows led to infinite subfolder duplication via a stray shortcut file. How can I avoid this? Can consciousness simply be a brute fact connected to some physical processes that dont need explanation? @media(min-width:0px){#div-gpt-ad-codevscolor_com-medrectangle-4-0-asloaded{max-width:336px!important;max-height:280px!important}}if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[336,280],'codevscolor_com-medrectangle-4','ezslot_4',153,'0','0'])};__ez_fad_position('div-gpt-ad-codevscolor_com-medrectangle-4-0');@media(min-width:0px){#div-gpt-ad-codevscolor_com-medrectangle-4-0-asloaded{max-width:336px!important;max-height:280px!important}}Lets use one for-in loop and try to remove all elements smaller than 4 using remove : @media(min-width:0px){#div-gpt-ad-codevscolor_com-box-4-0-asloaded{max-width:300px!important;max-height:250px!important}}if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[300,250],'codevscolor_com-box-4','ezslot_5',160,'0','0'])};__ez_fad_position('div-gpt-ad-codevscolor_com-box-4-0');@media(min-width:0px){#div-gpt-ad-codevscolor_com-box-4-0-asloaded{max-width:300px!important;max-height:250px!important}}It will print: The problem is that the iterator doesnt know if an item is removed from the list. @dizcza In python, every single bit of memory is allocated from the heap. What its like to be on the Python Steering Council (Ep. So this solution skips elements. Why does Python modify the list which is out of the loop? As you can see in other answers, you are trying to modify the list while iterating over it. What is the smallest audience for a communication that has been deemed capable of defamation? "different from expected". I've had to solve this problem more than once in applications I write. What its like to be on the Python Steering Council (Ep. For further reading, I highly recommend. After the first item is handled, the iterator is pointing at index = 1, but because you've removed an item, the next item is now at index zero, so it will be skipped. rev2023.7.24.43543. The pointer to the element moves through the list to the next spot available. What would naval warfare look like if Dreadnaughts never came to be? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. Share. What is the difference between Python's list methods append and extend? Ways to Iterate over a list in Python There are multiple ways to iterate over a list in Python. Of course, you can. Process 2 ( P2) iterates over POST_QUEUE via a for-loop at set intervals . How do I split a list into equally-sized chunks? In fact, that's unidiomatic. Like that: The question is: are the both ways above allowed or only the second one is error-free? For example, if you want to add one to every element in a list of integers, this would not work: Indeed, ints are immutable objects. Method 1: Create a new list. What should I do after I found a coding mistake in my masters thesis? Python3 from itertools import groupby test_list = [1, 4, 4, 4, 5, 6, 7, 4, 3, 3, 9] How to use built-in function pop in python? Agreed, iterating over a list to remove elements from it is not the clean way. Thanks for contributing an answer to Stack Overflow! python - How do I delete items from a dictionary while iterating over Does glide ratio improve with increase in scale? I know you should not add/remove items while iterating over a list. Python: Removing list element while iterating over list. Maybe you can express your desires with a while? Reason not to use aluminium wires, other than higher resitance, minimalistic ext4 filesystem without journal and other advanced features. This post provides an overview of some available alternatives to avoid this non-deterministic behavior. Remove items from a Python list while iterating it | Techie Delight This doesn't let the loop ever see 3. Why does list.remove() not work in this for-loop? Then the for loop goes to the second item in the list, which is not 2, but 3! One of the easiest and safest ways to remove elements from a list while iterating is to create a new list with only the elements you want to . How difficult was it to spoof the sender of a telegram in 1890-1920's in USA? The one-stop solution is to note down those unwanted elements and remove them in the next iteration. Modifying list while iterating [duplicate], Strange result when removing item from a list while iterating over it in Python, What its like to be on the Python Steering Council (Ep. Fortunately, in Python, you can simply alter the list in-place by assigning to the slice [:]. Python then picks the item at position 2 from the list (appearing to skip an item). python - Modifying list while iterating - Stack Overflow e.g. How to remove items from a list while iterating? - W3docs Here's a visual explanation of what is happening. we are iterating and also removing items simultaneously. You can use a list comprehension or generator expression to remove the unwanted items as shown above. rev2023.7.24.43543. May I reveal my identity as an author during peer review? I'll leave this up as a slight alternative. What should I do after I found a coding mistake in my masters thesis? I'm seeing this very late, but this answer is wrong. By using this site, you agree to the use of cookies, our policies, copyright terms and other conditions. You're modifying the list while you iterate over it. The second option wouldn't work as you think it will (especially since you want to delete the items of list1). This can result in incorrect results due to skipped elements, or an exception will be thrown to avoid non-deterministic behavior at a later stage. Another very interesting side-effect error. How do I figure out what size drill bit I need to hang some ceiling hooks? @dizcza The documentation would also not mention that mutating the size is allowed or not. For removing items, list.remove() method is used. I would just initialise a new list outside the loop and add items to it accordingly. To remove elements from a list while iterating over it, you need to go backwards: if delLater == True: for x in schedule[-1::-1]): if 'DELETE' in x: schedule.remove(x) A better option is to use a list-comprehension: We can loop through a list in Python using the 8 different methods. Why does Python skip elements when I modify a list while iterating over it? This would translate to a simple code below: Thats all about removing items from a dictionary while iterating over it in Python. Or please state your real problem. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. filter() creates one new list and we are assigning it to the variable. June 3, 2023 This answer was originally written in response to a question which has since been marked as duplicate: Removing coordinates from list on python There are two problems in your code: 1) When using remove (), you attempt to remove integers whereas you need to remove a tuple. remove list iteration When removing items from a list while iterating over the same list, a naive solution using list.remove can cause the iterator to skip elements: >>> lst = [1, 2, 3, 4, 5, 6, 7, 8] >>> for x in lst: if x < 6: lst.remove(x) >>> lst [2, 4, 6, 7, 8] Connect and share knowledge within a single location that is structured and easy to search. @variable It isn't a good idea. Use a secondary list to store the items you want to act upon and execute that logic in a loop after your initial loop. We and our partners use data for Personalised ads and content, ad and content measurement, audience insights and product development. If you believe this to be in error, please contact us at team@stackexchange.com. 1 Dumb observation: Your whole loop is kind of pointless if you're just looking for a specific key. Does this definition of an epimorphism work? Python does list iteration by index, so yes, deleting items (except from the end, using a negative slice) during iteration is potentially dangerous behavior: modifying them in place though is entirely safe. Is not listing papers published in predatory journals considered dishonest? Manage Settings Absolutely. remove items when looping/iterating a list. However, this can lead to unexpected results and errors if not done correctly. What is the most accurate way to map 6-bit VGA palette to 8-bit? Here's an example of creating a new list: Here's an example of using a range of indices to remove items: It is important to note that in python version 3.3 and later, for-loops in python internally make an iterator out of the input. Traversing a copy of the list works for the remove() method, as Chris Jester-Young suggested. Journey with Code and DesignCodeVsColor on TwitterAboutPrivacy PolicyT&CContact, Python index method to get the index of an item in a list, List all the files in a Zip file using Python 3, Python program to convert one string to JSON, Python program to read and write JSON from a file. The first way is to make a copy of the list, iterate on the newly created list and call remove on the original list. The easiest and most intuitive way to remove elements from a list is to create a new list and copy over all the elements except for the ones to be removed. (26 answers) Closed last year. So in the case of the Size being 5enter code here. This time it worked. Who counts as pupils or as a student in Germany? There are two separate processes running in Python script. To downvoter: Reason for downvote? Enter your email address to subscribe to new posts. I tried the following code but it didn't do it: list(x) is more pythonic than x[:] when you need a list copy. New comments cannot be posted and votes cannot be cast. Iterating over a list of objects in Python to access and change them is a common thing to do while coding. 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. Read our. So in python 3.3 and later, the following will raise a RuntimeError: dictionary changed size during iteration. You shouldn't modify a list while you iterate over it. I don't believe this is a problem. Are there any practical use cases for subtyping primitive types? Never alter the container you're looping on, because iterators on that container are not going to be informed of your alterations and, as you've noticed, that's quite likely to produce a very different loop and/or an incorrect one. I'm iterating over a list of elements in Python, do some action on it, and then remove them if they meet certain criteria. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. Why is a dedicated compresser more efficient than using bleed air to pressurize the cabin? In normal cases, looping on a copy of the container helps, but in your case it's clear that you don't want that, as the container will be empty after 50 legs of the loop and if you then try popping again you'll get an exception. To learn more, see our tips on writing great answers. Deleting a List Item While Iterating - Python FTW: Under the Hood You are not modifying the list, so to speak. (25 answers) Is there a simple way to delete a list element by value? minimalistic ext4 filesystem without journal and other advanced features. Python List: How to append objects while being used by a function? Why does CNN's gravity hole in the Indian Ocean dip the sea level instead of raising it? When I say bind the name list2 to the same object as list1 I was assuming that OP would know that list2 and list1 are both names for the same object. Python 3.0 range() now behaves like xrange() used to behave, except it works with values of arbitrary size. Use a while loop that checks for the truthfulness of the array: If you want list2 to have the same values as list1 you can just do: And if after that you want to use list1 for something else, you can just do: Bind the name list2 to the the same object as list1: (Note that if you modify the contents of list1, list2 will be change accordingly.). This is why you are only handling every other item. python - iterating through a list removing items, some items are not When contacting us, please include the following information in the email: User-Agent: Mozilla/5.0 _iPad; CPU OS 15_5 like Mac OS X_ AppleWebKit/605.1.15 _KHTML, like Gecko_ GSA/219.0.457350353 Mobile/15E148 Safari/604.1, URL: stackoverflow.com/questions/6500888/removing-from-a-list-while-iterating-over-it. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. How to remove items from a list while iterating? Python: Remove elements from a list while iterating - thisPointer Conclusions from title-drafting and question-content assistance experiments How to remove empty values from an array in python, Strange result when removing item from a list while iterating over it in Python, Weird behavior removing elements from a list in a loop in Python. What would naval warfare look like if Dreadnaughts never came to be? How do I clone a list so that it doesn't change unexpectedly after assignment? This might cause issues when there are other references to the list which need updating. But it runs with very a bad efficiency and the code is hard to maintain. You are simply modifying the elements in the list. 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. Iterating over list but some values are skipped? Need find the intersection using For Loop code in python? Iterating over dictionaries using 'for' loops. Try [*range (100)]. How can I randomly select an item from a list? Can a Rogue Inquisitive use their passive Insight with Insightful Fighting? This behavior is properly explained in this question. How do I split a list into equally-sized chunks? To learn more, see our tips on writing great answers. (or print functions in Python 3). How difficult was it to spoof the sender of a telegram in 1890-1920's in USA? Syntax: Iterator > iterator = map.entrySet ().iterator (); while (iterator.hasNext ()) { Map.Entry entry = iterator.next (); if (valueToBeRemoved.equals (entry.getValue ())) { iterator.remove (); } } Below is the implementation of the above approach: // Java program to remove an entry using value // from a HashMap while iterating over it Do NOT follow this link or you will be banned from the site. What is the smallest audience for a communication that has been deemed capable of defamation? 6:13 when the stars fell to earth? [Question] Removing objects from a list while iterating over it There are several ways to remove elements from the list while iterating some of them are: Using for in loop Using List Comprehension Using filter () function Method #1:Using for in loop To accomplish this, we must first make a copy of the list, and then iterate over that copied list.
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