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.

Each space separated value is a separate search term.
Terms are ANDed together.
[Brackets] indicate a tag term.
is:draft shows only draft posts.
is:community shows only community posts.

1 - 100 of 137

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 dictionary reference mutable 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 copy mutable dict 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 IndexError append 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.
Choosing the function to call depending on some condition function dispatch Problems typically being solved by using a dispatch dict.
Class attributes vs instance attributes dictionary mutable list class Class attributes are shared across all instances of a class; especially when using mutable containers.
collections.defaultdict’s “first argument must be callable” Community dictionary defaultdict The defaultdict requires a callable first argument, not the desired default value itself
Comparing strings as numbers `'12' < '2'`, and `min(['12', '2']) == '12'`
Comprehensions in a class body comprehension scope class Generator expressions and list/set/dict comprehensions create a new scope that can’t see class variables.
Confusing the Python command line interpreter REPL with the terminal shell command-line console shell repl Users trying to put shell commands into the Python REPL will get NameError or SyntaxError exceptions.
Convert a project to an executable file install pyinstaller cx_freeze py2exe installer exe windows executable Create a single executable (exe) from a script, package, or other project. A common question when using Windows.
Converting a floating point number to hexadecimal representation? hex float
Count number of each character in a string string counter dict Given a string `s`, what is the best way to count the occurences of the individual characters `c` in the string?
Determine the encoding of text decode encode charset string encoding bytes str chardet Given a byte string, determine what charset should be used to decode it to text.
*Duplicating a list through multiplication (``), shares references for the contents** multiplication mutable 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.
Dynamically getting or setting multiple variables variables How do I create a variable with a given name? OR, how do I create a lot of numbered variables? Or, how should I add stuff to the `globals` dict? And the inverse, given a string with a variable name, how do I get the variable?
Extract text from a file between two markers file extraction Having a file with some `<START>` and some `<END>` marker, extract all the content between those markers.
FileNotFoundError from os.walk() FileNotFoundError os.walk path open The OP is surprised that attempting to access a file returned by `os.walk()` fails, not realizing that you have to add the directory back on.
FileNotFoundError on relative path FileNotFoundError path open Using `open("relative/path")` raises `FileNotFoundError` even though the file exists
File not found/Windows file error with Windows path windows path Windows pathnames in source code with backslashes are fun.
File not found with os.listdir(dir) listdir path listdir() returns just the file names; you have to `os.path.join()` back the directory name
Flattening a nested for loop / dynamic nested loops itertools nested for-loop When you have multiple for loops, one nested in another, or you need a dynamic number of nested loops, use `itertools.product()`.
Flattening a nested sequence list flatten
For loop contains an unconditional return return for-loop newbie The asker is surprised that their `for` loop containing a `return` statement exits after one iteration
For loop not working twice on the same file descriptor file The OP’s code attempts to use `for` loops two or more times to re-read a file without rewinding it.
Formatting a float to remove superfluous zeros string float format
Get attribute by string name string attribute `name = 'asdf'; foo.name` isn’t the same as `foo.asdf`. You need `getattr`.
Global variables are not properly updated/visible across functions local global The user attempts to read/write global variables but does not correctly use the `global` keyword
Grouping together contiguous runs of integers from a list
How do comparisons such as `2 == 2 > 1` work? operator_chaining chaining
How does string comparison work? string comparison People get confused when `'longer string' < 'short string'`.
How does string or list slicing work? extract slicing string list slice extraction How can I extract different parts of a string or list using slicing notation or the `slice()` built-in?
How do I access items of my dict using the dot operator? dict
How do I do a loading animation or progress bar on the command line? loading progress animation
How do I get the value from a multiply-nested dict, given a collection of keys? dict
How do I read keyboard input without waiting for the user to press Enter? input keyboard
How do I unpack the contents of an iterable into separate variables? unpacking assignment Unpacking assignment from a list, tuple, map, etc. into separate names
How do I use non-ASCII strings in my Python script? string encoding unicode coding character character-set Python cannot tell out of the box which character set your script uses, so you have to tell it. The error you get is `SyntaxError: Non-ASCII character '\xe2' in file bla.py on line 1, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details`
How to annotate generic containers generic annotation typing type-hint Specifying the type of items contained within a homogeneous list (or other collection) for the purpose of type hinting in PyCharm and other IDEs
How to generate random numbers that are different random
How to use pip corresponding to a given version of python? version pip There are multiple installations of python, and neither `pip` nor `pip3` corresponds to the version OP wants.
Indentation problems caused by using tabs, with the tab size in the editor configured to 4 spaces. indentation tabs Python 2 and 3 allow tabs to be used to indent lines. Python 2 will expand these to a multiple of 8, in Python 3 a tab can be equal only to another tab;, but many editors and Stack Overflow expand tabs to multiples of 4 instead. This can lead to subtle and weird flow errors.
Indentation troubleshooting (IndentationError, TabError) indentation tabs There is a broad array of possible indentation errors, and a broader array of fixes Examples: IndentationError: unexpected indent IndentationError: expected an indented block TabError: inconsistent use of tabs and spaces in indentation SyntaxError (when connected to optional sections for compound statements broken by indentation errors)
Infinite recursion in `__getattribute__` recursion attribute The questioner’s `__getattribute__` includes `self.something`, triggering `__getattribute__` again.
`input` in Python 3.x and the loss of `raw_input` string raw_input input With Python 3.x `raw_input` was scrapped and `input` now always returns a string object.
Input not returning a number string input number `input` (Python 3.x) and `raw_input` (Python 2.x) return the variable as a string, this can confuse users when they expect their “number” to be a number.
`input` raises `NameError` input In Python 2, `input` treats strings as variable names and raises a `NameError`.
Input value isn’t sent when submitting form html input form flask Browsers only send the values of inputs that have a `name` attribute.
Integer division returning “incorrect” values division integer Integer division (in Python 2.x) can cause issues for those unaware that `1/2 != 0.5`
Is it more efficient to use a tuple/list/set for membership testing tuple internals list
`is` operator works confusingly with integers is operator integer 256 is 256, 257 is not 257
Iterating over a list and indexing with the result leads to TypeError index list iterate `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.
Jinja escapes JSON so it is invalid JavaScript javascript flask escape jinja The user dumps some data to JSON and renders it in a template, but the data contains HTML escape characters so it’s invalid JavaScript.
main() function doesn’t run when the script is run main newbie main() function defined but never called
Mapping a range of numbers onto a different range whilst preserving order and relative distance? range
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.
Module shadowing. AttributeError: module ‘object’ has no attribute ‘foobar’ python module shadowing exceptions Make sure you are not name-clashing your modules
Multiple `await`s run their targets one after the other, not concurrently asyncio await async The code has several `await a`, `await b`, … and executes `b` after `a`. The user expected `a` and `b` to run at the same time.
Mutable Default Arguments (& the Principle of “Least Astonishment”) function argument mutable “Astonishing” behaviour of default mutable arguments: `def cabbage(leaves=[])`.
Name-mangling the class attribute att to _ClassNameatt? oop name-mangling Name-mangling in Python is one of the few ways to have “private” attributes. Note that within Python it is generally held that “we’re all consenting adults” and so there is no way to absolutely guarantee private attributes.
Negative number raised to an even power returns a negative number math power A negative number raised to an even power returns a negative number due to the operator precedence.
‘NoneType’ object has no attribute whatever none attribute object Usually, this means a function returned `None` where you expected it to return an object of some sort.
Opening files with relative paths cwd relative absolute open `open('some file')` for files located next to the module source, will fail when the current working directory does not match expectations. The solution is to use `__file__` to construct an absolute path.
Order of Python dictionaries/sets set dict
Output string without quotes string repr >>> 'value' 'value' Shell or repr returns string with quotes.
Passing a single argument to `cursor.execute()`, or the “missing comma” comma tuple sql A single string or other object, possibly in parentheses, is passed as the second argument to `execute()`.
Passing a variable by reference variables
Photoimages don’t appear on Tkinter canvas
Pretty printing a table of data WARNING : Python 2.4 eta post, doesn’t discuss `str.format`
Printing bytes produces different output than printing string string bytes python3 print Printing bytes still shows the `b` prefix, escapes, and doesn’t break on newlines.
Printing without adding a newline or whitespace at the end print
print statement SyntaxError in Python 3 SyntaxError print Starting with Python 3.x print changed from a statement to a function.
Print without space in between values print How to output `Value is "42"`, yet `print 'Value is: "', 42, '"'`, or `print('Value is: "', 42, '")'` adds extra spaces around `42`
Programs using `multiprocessing` hang (deadlock) and never complete multiprocessing deadlock hang Typical cause: attempting to `join()` before performing `get()` on cross-process queues.
Prompting the user for input until you get a valid response input prompt It’s sometimes necessary to prompt the user for a response and check it, then re-prompt until it fulfils some test.
PyCharm unresolved references pycharm The asker sees “unresolved reference” errors in PyCharm even though the library is installed.
Python division - difference between `/` and `//`. division integer Division can sometimes trip up the new user, especially the difference between Python 2.x and 3.x.
Python http client fails to load a page the browser succeeds in loading http urllib2 requests urllib Someone using `requests` or `urllib.client` or `urllib2`, etc. is trying to load a URL but they get a different result from what their browser shows. Either because they get an HTTP error (404, 500, etc.) or different content from what they expected.
Regex and HTML regex html parsing Tony the Pony…he comes…
Removing items from a list while iterating over the list remove iteration list How to remove items from a list while iterating over the same list without skipping elements.
Render string in Jinja to JavaScript variable javascript flask jinja json Rendering a string directly results in an invalid JavaScript value. Need to use the `\|tojson` filter.
Return JSON response from Flask view flask json Flask provides the `jsonify` method to convert data into a JSON response.
Reverse a string string reverse How do you reverse a string? e.g. `"foobar"[::-1]`
Same id for multiple (short-lived objects); memory address reused address id cpython object 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?
Selection with .loc in Pandas indexing pandas A Pandas indexing how-to.
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.
Sort a dictionary sort dict
Sorting a list based on special / multiple criteria / keys sort key list sorting
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. chunk split list How to split a single list into multiple lists with a fixed (maximum) size?
strip() strips too much string strip People get surprised when str.strip treats the argument as a set of characters to remove.
The truth value of an array with more than one element is ambiguous. pandas numpy The caller gets a `ValueError` from using an array, frame, or series in a boolean context.
Thread(target=func()) runs the function in the wrong thread multithreading multiprocessing threading The questioner’s code runs a function and makes its return value a thread’s target instead of using the function as the target directly.
Trailing comma produces a tuple, not the intended object comma tuple assignment Adding a trailing comma can cause problems, primarily the assignment of a tuple rather than the intended object.
Trying to access the n-th element of a list gives ValueError when using .index() index valueerror list I have a `list` of n-many objects - why does `list.index(0)` raise a `ValueError` and not return the first item?
Type-Annotating a classmethod classmethod typing annotations An `@classmethod` is used to create instance and type annotations must express the precise class.
TypeError: cannot concatenate ‘str’ and ‘int’ objects string concatenation int

1 - 100 of 137