Questions tagged [python]
Python is a dynamically typed, multi-purpose programming language. It is designed to be quick to learn, understand, and use, and enforces a clean and uniform syntax. Please note that Python 2 is officially out of support as of 2020-01-01. For version-specific Python questions, add the [python-2.7] or [python-3.x] tag. When using a Python variant (e.g. Jython, PyPy) or library (e.g. Pandas, NumPy), please include it in the tags.
2,167,599
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What does the "yield" keyword do in Python?
What is the use of the yield keyword in Python? What does it do?
For example, I'm trying to understand this code1:
def _get_child_candidates(self, distance, min_dist, max_dist):
if self._leftchild ...
- 144k
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votes
46
answers
4.5m
views
What does if __name__ == "__main__": do?
What does this do, and why should one include the if statement?
if __name__ == "__main__":
print("Hello, World!")
If you are trying to close a question where someone should be ...
- 178k
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votes
32
answers
2.8m
views
Does Python have a ternary conditional operator?
Is there a ternary conditional operator in Python?
- 178k
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7029
votes
40
answers
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views
How do I check whether a file exists without exceptions?
How do I check whether a file exists or not, without using the try statement?
- 77.3k
6809
votes
43
answers
3.2m
views
How do I merge two dictionaries in a single expression in Python?
I want to merge two dictionaries into a new dictionary.
x = {'a': 1, 'b': 2}
y = {'b': 3, 'c': 4}
z = merge(x, y)
>>> z
{'a': 1, 'b': 3, 'c': 4}
Whenever a key k is present in both ...
- 122k
6035
votes
66
answers
4.5m
views
How do I execute a program or call a system command?
How do I call an external command within Python as if I had typed it in a shell or command prompt?
- 62.1k
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27
answers
3.6m
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How do I create a directory, and any missing parent directories?
How do I create a directory at a given path, and also create any missing parent directories along that path? For example, the Bash command mkdir -p /path/to/nested/directory does this.
- 103k
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votes
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Accessing the index in 'for' loops
How do I access the index while iterating over a sequence with a for loop?
xs = [8, 23, 45]
for x in xs:
print("item #{} = {}".format(index, x))
Desired output:
item #1 = 8
item #2 = ...
- 317k
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votes
33
answers
4.1m
views
How do I make a flat list out of a list of lists?
I have a list of lists like
[
[1, 2, 3],
[4, 5, 6],
[7],
[8, 9]
]
How can I flatten it to get [1, 2, 3, 4, 5, 6, 7, 8, 9]?
If your list of lists comes from a nested list ...
- 52.9k
4584
votes
36
answers
1.1m
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@staticmethod vs @classmethod in Python
What is the difference between a method decorated with @staticmethod and one decorated with @classmethod?
- 144k
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38
answers
2.9m
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How slicing in Python works
How does Python's slice notation work? That is: when I write code like a[x:y:z], a[:], a[::2] etc., how can I understand which elements end up in the slice? Please include references where appropriate....
- 78.8k
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answers
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views
Finding the index of an item in a list
Given a list ["foo", "bar", "baz"] and an item in the list "bar", how do I get its index 1?
- 47.6k
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votes
17
answers
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Iterating over dictionaries using 'for' loops
d = {'x': 1, 'y': 2, 'z': 3}
for key in d:
print(key, 'corresponds to', d[key])
How does Python recognize that it needs only to read the key from the dictionary? Is key a special keyword, or is ...
- 43.9k
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votes
33
answers
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How to iterate over rows in a DataFrame in Pandas
I have a pandas dataframe, df:
c1 c2
0 10 100
1 11 110
2 12 120
How do I iterate over the rows of this dataframe? For every row, I want to access its elements (values in cells) by the name ...
- 125k
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votes
25
answers
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Using global variables in a function
How do I create or use a global variable inside a function?
How do I use a global variable that was defined in one function inside other functions?
Failing to use the global keyword where appropriate ...
- 154k
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votes
54
answers
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views
3768
votes
6
answers
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views
Catch multiple exceptions in one line (except block)
I know that I can do:
try:
# do something that may fail
except:
# do this if ANYTHING goes wrong
I can also do this:
try:
# do something that may fail
except IDontLikeYouException:
# ...
- 110k
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votes
21
answers
3.5m
views
3691
votes
24
answers
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views
Convert bytes to a string in Python 3
I captured the standard output of an external program into a bytes object:
>>> from subprocess import *
>>> stdout = Popen(['ls', '-l'], stdout=PIPE).communicate()[0]
>>> ...
- 42.8k
3643
votes
28
answers
958k
views
What is the difference between __str__ and __repr__?
What is the difference between __str__ and __repr__ in Python?
- 115k
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14
answers
2.3m
views
3587
votes
10
answers
6.5m
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Does Python have a string 'contains' substring method?
I'm looking for a string.contains or string.indexof method in Python.
I want to do:
if not somestring.contains("blah"):
continue
- 260k
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votes
21
answers
5.2m
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How can I add new keys to a dictionary?
How do I add a new key to an existing dictionary? It doesn't have an .add() method.
- 49.6k
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votes
21
answers
7.9m
views
How do I list all files of a directory?
How can I list all files of a directory in Python and add them to a list?
- 44.9k
3414
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34
answers
5.1m
views
How do I sort a dictionary by value?
I have a dictionary of values read from two fields in a database: a string field and a numeric field. The string field is unique, so that is the key of the dictionary.
I can sort on the keys, but how ...
- 42.5k
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votes
17
answers
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views
How do I select rows from a DataFrame based on column values?
How can I select rows from a DataFrame based on values in some column in Pandas?
In SQL, I would use:
SELECT *
FROM table
WHERE column_name = some_value
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votes
34
answers
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"Least Astonishment" and the Mutable Default Argument
Anyone tinkering with Python long enough has been bitten (or torn to pieces) by the following issue:
def foo(a=[]):
a.append(5)
return a
Python novices would expect this function called with ...
- 139k
3324
votes
28
answers
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views
What does ** (double star/asterisk) and * (star/asterisk) do for parameters?
What do *args and **kwargs mean in these function definitions?
def foo(x, y, *args):
pass
def bar(x, y, **kwargs):
pass
See What do ** (double star/asterisk) and * (star/asterisk) mean in a ...
- 34.6k
3289
votes
17
answers
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views
3264
votes
40
answers
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views
How do I pass a variable by reference?
I wrote this class for testing:
class PassByReference:
def __init__(self):
self.variable = 'Original'
self.change(self.variable)
print(self.variable)
def change(self, ...
- 48.5k
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votes
24
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How do I clone a list so that it doesn't change unexpectedly after assignment?
While using new_list = my_list, any modifications to new_list changes my_list every time. Why is this, and how can I clone or copy the list to prevent it?
- 65.1k
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31
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How do I concatenate two lists in Python?
How do I concatenate two lists in Python?
Example:
listone = [1, 2, 3]
listtwo = [4, 5, 6]
Expected outcome:
>>> joinedlist
[1, 2, 3, 4, 5, 6]
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How do I check if a list is empty?
For example, if passed the following:
a = []
How do I check to see if a is empty?
- 187k
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votes
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answers
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answers
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Understanding Python super() with __init__() methods [duplicate]
Why is super() used?
Is there a difference between using Base.__init__ and super().__init__?
class Base(object):
def __init__(self):
print "Base created"
class ChildA(...
- 51.2k
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votes
11
answers
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views
Manually raising (throwing) an exception in Python
How do I raise an exception in Python so that it can later be caught via an except block?
- 261k
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votes
14
answers
5.9m
views
How do I change the size of figures drawn with Matplotlib?
How do I change the size of figure drawn with Matplotlib?
- 37.6k
3157
votes
16
answers
2.9m
views
How can I access environment variables in Python?
How can I get the value of an environment variable in Python?
- 32.8k
3133
votes
21
answers
645k
views
How do I make function decorators and chain them together?
How do I make two decorators in Python that would do the following?
@make_bold
@make_italic
def say():
return "Hello"
Calling say() should return:
"<b><i>Hello</i>&...
- 87.4k
3120
votes
65
answers
2.4m
views
How do I print colored text to the terminal?
How do I output colored text to the terminal in Python?
- 31.4k
3111
votes
20
answers
3.3m
views
What is the difference between Python's list methods append and extend?
What's the difference between the list methods append() and extend()?
- 224k
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answers
1.6m
views
How do I split a list into equally-sized chunks?
How do I split a list of arbitrary length into equal sized chunks?
See also: How to iterate over a list in chunks.
To chunk strings, see Split string every nth character?.
- 32.5k
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votes
12
answers
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views
Why is "1000000000000000 in range(1000000000000001)" so fast in Python 3?
It is my understanding that the range() function, which is actually an object type in Python 3, generates its contents on the fly, similar to a generator.
This being the case, I would have expected ...
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13
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Find the current directory and file's directory [duplicate]
How do I determine:
the current directory (where I was in the shell when I ran the Python script), and
where the Python file I am executing is?
- 61.2k
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Convert string "Jun 1 2005 1:33PM" into datetime
I have a huge list of datetime strings like the following
["Jun 1 2005 1:33PM", "Aug 28 1999 12:00AM"]
How do I convert them into datetime objects?
- 236k
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votes
36
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Renaming column names in Pandas
I want to change the column labels of a Pandas DataFrame from
['$a', '$b', '$c', '$d', '$e']
to
['a', 'b', 'c', 'd', 'e']
- 29.1k
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answers
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How can I remove a key from a Python dictionary?
I want to remove a key from a dictionary if it is present. I currently use this code:
if key in my_dict:
del my_dict[key]
Without the if statement, the code will raise KeyError if the key is not ...
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How to sort a list of dictionaries by a value of the dictionary in Python?
How do I sort a list of dictionaries by a specific key's value? Given:
[{'name': 'Homer', 'age': 39}, {'name': 'Bart', 'age': 10}]
When sorted by name, it should become:
[{'name': 'Bart', 'age': 10}, ...
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How do I install pip on Windows?
pip is a replacement for easy_install. But should I install pip using easy_install on Windows? Is there a better way?