Common Questions - sopython

This is a list of common questions within the Python tag. Each question includes some canonical SO questions that can be used to close-vote any new questions that match. If you have any suggestions then please come see us in chat.

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1 - 30 of 30

Accessing current and next item during iteration iteration list Sometimes it is useful to access both the current and next item when iterating over a list.
Appending in a loop creates a list of unexpected duplicates mutable dictionary reference list A mutable object is modified in a loop and appended to a list. The OP is surprised that all of the list items are duplicates of the last appended object.
Assigning a list / dict does not make a copy mutable dict copy list The asker does `new_list = old_list` or `new_dict = old_dict` and is surprised when changes to the one reference are reflected in the other. The fix is to copy the list or dict in one of many ways.
Attempting to append to a list by assigning to an element outside the range of the list leads to IndexError append IndexError list An `IndexError` is raised because the asker tried using `lst[len(lst)] = new_val` to append to a list. They should have used `lst.append` (and maybe `lst.extend`) instead.
Convert a project to an executable file exe install windows installer executable cx_freeze pyinstaller py2exe Create a single executable (exe) from a script, package, or other project. A common question when using Windows.
*Duplicating a list through multiplication (``), shares references for the contents** mutable multiplication list Multiplying a list creates a shallow copy, not a deep copy, so any mutable objects in the list are then shared. Solution is to use a list comprehension to create separate objects.
File not found with os.listdir(dir) path listdir listdir() returns just the file names; you have to `os.path.join()` back the directory name
Grouping together contiguous runs of integers from a list
How does string or list slicing work? extraction extract slicing slice string list How can I extract different parts of a string or list using slicing notation or the `slice()` built-in?
How do I unpack the contents of an iterable into separate variables? assignment unpacking Unpacking assignment from a list, tuple, map, etc. into separate names
How to annotate generic containers generic type-hint annotation typing Specifying the type of items contained within a homogeneous list (or other collection) for the purpose of type hinting in PyCharm and other IDEs
Iterating over a list and indexing with the result leads to TypeError iterate index list `TypeError: list indices must be integers or slices, not ...`
Iterating over pairs of consecutive items in a list
Iterating over two (or more) sequences in parallel iteration zip Zip can be used to iterate over multiple lists at the same time. In Python 3.x a generator is used whilst in Python 2.x a new list of tuples is created, as such itertools.izip may be a better choice.
Modifying a dictionary while iterating over it iteration dict Python throws an error if you modify the `dict` you are currently iterating over. The obvious workaround is to copy the keys to a separate list, or collecting the keys to change into a separate list and modifying them separately after the loop.
Mutable Default Arguments (& the Principle of “Least Astonishment”) mutable argument function “Astonishing” behaviour of default mutable arguments: `def cabbage(leaves=[])`.
Passing a single argument to `cursor.execute()`, or the “missing comma” tuple sql comma A single string or other object, possibly in parentheses, is passed as the second argument to `execute()`.
Removing items from a list while iterating over the list iteration remove list How to remove items from a list while iterating over the same list without skipping elements.
Return JSON response from Flask view flask json Flask provides the `jsonify` method to convert data into a JSON response.
Same id for multiple (short-lived objects); memory address reused cpython id object address Why does id({}) == id({}) and id([]) == id([]) in CPython?
Saving and loading data save Why do my variables disappear when I quit and restart? What format should I use for saving?
Single column query results in a list of tuples in SQLAlchemy sqlalchemy tuple The result set of a single column query is a list of tuples, instead of a list of values.
Sorting a list based on special / multiple criteria / keys sort sorting key list
Sorting / comparing version strings sort version Sorting a list of version strings, eg `["1.1.2", "1.0.0", "1.3.3", "1.0.12", "1.0.2"]`, into natural order.
Splitting a list into even chunks. split chunk list How to split a single list into multiple lists with a fixed (maximum) size?
Trying to access the n-th element of a list gives ValueError when using .index() valueerror index list I have a `list` of n-many objects - why does `list.index(0)` raise a `ValueError` and not return the first item?
Type Error: “'types.GenericAlias' object is not …” after using [] instead of () on a container type typo annotations generic A typo like `list["abcd"]` instead of `list("abcd")` leads to `Type Error: “'types.GenericAlias' object is not ...”`
Using a list comprehension with str.join join string list-comprehension When using `str.join` a list comprehension (rather than a generator expression) provides a significant speed boost
What is a for loop doing in a list / dictionary / set (question looking for comprehension syntax) name-that-syntax list-comprehension dict-comprehension The terminology makes finding a canonical “That’s a list / dict / etc. comprehension” question hard.
Why is it a mistake to assign the result of list.append()? append None list

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