- Python default language cause problem on txt write how to#
- Python default language cause problem on txt write code#
Instead, create a new list that copies all the items except the ones you want to delete, and then replace the original list. įigure 8-2: When the loop removes 'mello', the items in the list shift down one index, causing i to skip over 'yello'. The 'yello' string slipped by unexamined! Don’t remove items from a list while you’re iterating over that list. The next iteration of the loop examines index 3, which is now the last 'hello', as in Figure 8-2. But this shifted all the remaining items in the list down one index, moving 'yello' from index 3 to index 2. The reason is that when the for loop was examining index 2, it deleted 'mello' from the list.
It seems that 'yello' is left in the list. if word != 'hello': # Remove everything that isn't 'hello'. The naive approach is to iterate over the list, deleting the items that don’t match 'hello': > greetings =
Python default language cause problem on txt write code#
Consider code in which we want to remove any string that isn’t 'hello' from a list. Similarly, you shouldn’t delete items from a list while iterating over it. You now have a clothes list with matching socks. Then, after the loop, we modify clothes by extending it with the contents of newClothes. Instead, it changed a separate list, newClothes. Our for loop iterated over the items in the clothes list but didn’t modify clothes inside the loop. > clothes.extend(newClothes) # Appends the items in newClothes to clothes.Ī visualization of the execution of this code is at. newClothes.append(clothing) # We change the newClothes list, not clothes. Instead, use a separate list for the contents of the new, modified list, such as newClothes in this example: > clothes = The takeaway is don’t add items to a list while you’re iterating over that list. The loop only stops once the computer runs out of memory and crashes the Python program or until you interrupt it by pressing Ctrl-C.įigure 8-1: On each iteration of the for loop, a new 'red sock' is appended to the list, which clothing refers to on the next iteration. This will continue happening, as shown in Figure 8-1, which is why we see the never-ending stream of 'Added a sock.' messages. This makes the list, giving the list another string for Python to iterate over. The for loop reaches the second 'red sock' on the next iteration, so it appends another 'red sock' string. The problem is that when you append 'red sock' to the clothes list, the list now has a new, third item that it must iterate over. You’ll find a visualization of the execution of this code at. print('Added a sock:', clothing) # Inform the user. clothes.append(clothing) # Add the sock's pair. if 'sock' in clothing: # Find strings with 'sock'. > for clothing in clothes: # Iterate over the list. It gets caught in an infinite loop, and you’ll have to press Ctrl-C to interrupt it: > clothes = The task seems straightforward: iterate over the list’s strings, and when you find 'sock' in a string, such as 'red sock', append another 'red sock' string to the list.īut this code won’t work.
Consider this scenario: you want to iterate over a list of strings that describe items of clothing and ensure that there is an even number of socks by inserting a matching sock each time a sock is found in the list. Don’t Add or Delete Items from a List While Looping Over ItĪdding or deleting items from a list while looping (that is, iterating) over it with a for or while loop will most likely cause bugs.
Python default language cause problem on txt write how to#
This chapter informs you how to avoid these common gotchas. And you must use a trailing comma when you write tuples that contain a single item. The inequality operator != has unusual behavior when you chain them together. You’ll learn how the sort() method doesn’t sort items in an exact alphabetical order and how floating-point numbers can have rounding errors. This chapter explains how mutable objects, such as lists and dictionaries, can behave unexpectedly when you modify their contents. Knowing the programming lore behind these gotchas can help you understand why Python behaves strangely sometimes. New Python programmers must learn to avoid some common “gotchas.” Programmers learn this kind of knowledge randomly, from experience, but this chapter collects it in one place. Every language has warts (some more than others), and Python is no exception. Although Python is my favorite programming language, it isn’t without flaws.
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