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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101 - 137 of 137

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 ...”`
TypeError: ‘X’ object is not callable module callable shadowing list You get this error when you try to call `X()` when `X` isn’t a function, method, or class.
Type hint forward references type-hinting annotations A type hint that tries to use a not-yet-defined name will throw a NameError.
UnboundLocalError: local variable ‘…’ referenced before assignment
UnicodeDecodeError: ‘charmap’ codec cannot decode … utf-8 windows unicode character-encoding Most commonly, this is a Windows user forgetting to put `encoding='utf-8'` when opening a file.
UnicodeDecodeError: ‘utf-8’ codec cannot decode … utf-8 character-encoding unicode User tries to read data from a file which is clearly not in UTF-8, but fails to understand that this is what the error message means.
Use Jinja variable in function call variable flask jinja When inside a Jinja expression, you reference the variables directly, rather than within nested curly braces.
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
Using an iterable to pass multiple arguments to a function. splat arguments function The splat operator (*) can be used to unpack variables from a container when calling a function.
Using `print` to “`return`” values fails with nesting functions newbie return print A function using `print` for the output seems to return it, but does not allow using the output.
Using reduce in Python 3.x functools reduce From Python 3 onwards reduce must be imported from functools.
Using the % symbol in old-style string formatting string format `%` can be used in string formatting. Note that it is now recommended to use the new `str.format()` syntax.
Weighted random selection with and without replacement random choice Choosing elements from a sequence with a non-uniform probability.
What does `__all__` do in python? module `__all__` defines the public objects within a module
What does (double star) and * (star) do for Python parameters?** unpack splat How does splat unpacking work for sequences () and dictionaries (*).
What does `if __name__ == '__main__'` mean?
What does the percent operator do when used on a string?
What does the @ symbol mean? Seeing something like `@contract.bind` indicates a decorator function in python.
What is a docstring? documentation docstring help A docstring is a the first unassigned string in a function, class, or module declaration.
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.
What is the difference between client-side and server-side programming? server client concept Why doesn’t my Python execute in this rendered JavaScript?
Why am I getting `None` after using `sort` or `reverse`? sort reverse None
Why are my tkinter variables `None` when I do `x = Button().pack()` or `.grid()`? tkinter None
Why can’t I add new attributes to an object? As in, `x = object(); x.foo = 23`? object
Why do backslashes appear twice? string The representation of a string with backslashes can sometimes throw a user as they are escaped.
Why does my function print the answer and then None? function None If a function does not return a value then it automatically returns `None`.
Why does my recursive function return `None`? function None The recursive call in a function misses a `return` and thus sometimes returns `None`.
Why does my Tkinter Button command execute early and only once? tkinter button
Why doesn’t 2.__add__(3) work in Python? dunder integer float Python parses 2. as the float 2.0, this can cause troubles when trying to use methods on integers.
Why doesn’t calling a Python string method (ex. replace), do anything unless you assign its output?
Why doesn’t `if x == 'A' or 'B' or 'C'` do what I expect? if-statement boolean Testing against multiple values using boolean operators, sometimes people think booleans work like English grammar.
Why do my lambda functions or nested functions created in a loop all use the last loop value when called? tkinter lambda callback The OP expects a variable in their callback lambdas to have the value it had when the function was created, instead it has the current value at calling time.
Why don’t binary operators work when I’m trying to use them as logical operators? operator and boolean logical or Coming from other languages people might try to use `&&` and `\|\|` as logical operators, which fails. The obvious solution is to use `&` and `\|` instead… and then it still “doesn’t work”, because they need `and` and `or`.
Why don’t str.lower / str.upper / str.title / str.replace / str.strip work? immutable string Strings are immutable, so methods on the string that alter the string return a new value. Typically beginners forget this and call str.upper() or str.lower() or str.title() or str.replace() or str.strip() without assigning the new string back to the variable.
Why is it a mistake to assign the result of list.append()? append None list
Why is this particular floating point math operation not giving the correct answer? math multiplication addition division floating-point subtraction float It’s a hardware issue.
Writing CSV adds blank lines between rows csv newline The way Python handles newlines on Windows can result in blank lines appearing between rows when using `csv.writer`.

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