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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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.
Choosing the function to call depending on some condition dispatch function Problems typically being solved by using a dispatch dict.
Comparing strings as numbers `'12' < '2'`, and `min(['12', '2']) == '12'`
Determine the encoding of text encode str encoding charset bytes decode chardet string 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** 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.
FileNotFoundError on relative path path FileNotFoundError open Using `open("relative/path")` raises `FileNotFoundError` even though the file exists
Flattening a nested sequence flatten list
How do comparisons such as `2 == 2 > 1` work? operator_chaining chaining
How do I use non-ASCII strings in my Python script? coding encoding unicode character string 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`
Indentation problems caused by using tabs, with the tab size in the editor configured to 4 spaces. tabs indentation 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.
Input not returning a number input string 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`.
Integer division returning “incorrect” values integer division Integer division (in Python 2.x) can cause issues for those unaware that `1/2 != 0.5`
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.
Negative number raised to an even power returns a negative number power math A negative number raised to an even power returns a negative number due to the operator precedence.
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`
Python division - difference between `/` and `//`. integer division Division can sometimes trip up the new user, especially the difference between Python 2.x and 3.x.
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.
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 / 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?
Trailing comma produces a tuple, not the intended object tuple assignment comma Adding a trailing comma can cause problems, primarily the assignment of a tuple rather than the intended object.
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.
Why am I getting `None` after using `sort` or `reverse`? sort reverse None
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 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 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 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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