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Why does this list comprehension only "sometimes" work?


Why does this list comprehension only "sometimes" work?

By : Teo Jurado
Date : November 28 2020, 12:01 PM
it helps some times As Blckknght stated, in my second "working" example, I was incorrectly iterating over parsed_data2_hostnames instead of parsed_data2. Once I corrected this mistake, I verified I am getting the same keyerror issue.
Since my problem of parsing that json data is a completely separate issue, I'll close this. Blckknght.
code :


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What does "list comprehension" mean? How does it work and how can I use it?

What does "list comprehension" mean? How does it work and how can I use it?


By : Rasta Mouse
Date : March 29 2020, 07:55 AM
fixed the issue. Will look into that further From the documentation:
code :
>>> l = [] 
>>> for x in range(10):
...     l.append(x**2)
>>> l
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> list(map(lambda x: x**2, range(10)))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> [x**2 for x in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> [x.strip() for x in ('foo\n', 'bar\n', 'baz\n')]
['foo', 'bar', 'baz']
>>> [int(x) for x in ('1', '2', '3')]
[1, 2, 3]
>>> d = {'foo': '10', 'bar': '20', 'baz': '30'}
>>> [d[x] for x in ['foo', 'baz']]
['10', '30']
>>> d = {'foo': '10', 'bar': '20', 'baz': '30'}
>>> [int(d[x].rstrip('0')) for x in ['foo', 'baz']]
[1, 3]
>>> l = []
>>> for x in range(10):
...     if x%2:
...         l.append(x)
>>> l
[1, 3, 5, 7, 9]
>>> [x for x in range(10) if x%2]
[1, 3, 5, 7, 9]
>>> [i if i%2 != 0 else None for i in range(10)]
[None, 1, None, 3, None, 5, None, 7, None, 9]
>>> [i for x in [[1, 2, 3], [4, 5, 6]] for i in x]
[1, 2, 3, 4, 5, 6]

>>> [j for x in [[[1, 2], [3]], [[4, 5], [6]]] for i in x for j in i]
[1, 2, 3, 4, 5, 6]
>>> [j for j in x for x in [[1, 2, 3], [4, 5, 6]]]
Traceback (most recent call last):
  File "<input>", line 1, in <module>
NameError: name 'x' is not defined
>>> {x for x in [1, 1, 2, 3, 3, 1]}
{1, 2, 3}
>>> set([i for i in [1, 1, 2, 3, 3, 1]])
{1, 2, 3}
>>> {i: i**2 for i in range(5)}
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
>>> d = {}
>>> for i in range(5):
...     d[i] = i**2
>>> d
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
>>> (i for i in range(5))
<generator object <genexpr> at 0x7f52703fbca8>
>>> def gen():
...     for i in range(5):
...         yield i
>>> gen()
<generator object gen at 0x7f5270380db0>
>>> gen = (i for i in range(5))
>>> next(gen)
0
>>> next(gen)
1
>>> list(gen)
[2, 3, 4]
>>> next(gen)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
StopIteration
>>> sum(i**2 for i in range(5))
30
Python List Comprehension: Using "if" Statement on Result of the Comprehension

Python List Comprehension: Using "if" Statement on Result of the Comprehension


By : Mohan R
Date : March 29 2020, 07:55 AM
will help you Don't pass a list to filter, pass a generator expression, and it will only be iterated once:
code :
filter(None, (x.strip() for x in str_list))
[y for y in (x.strip() for x in str_list) if y]
How does a double "for" work in list comprehension?

How does a double "for" work in list comprehension?


By : Miguel
Date : March 29 2020, 07:55 AM
this one helps. That is a generator and has nothing to do with Pandas.
The term ([x, y] for x in q for y in p) is a Python generator. You can assign this to a variable, say g = ([x, y] for x in q for y in p) and then iterator over it:
code :
for element in g:
    print(element)
for x in q:
    for y in p:
        yield [x, y]
How to "convert" from a list-comprehension with both: for-in & if-in... to "normal" python3 code

How to "convert" from a list-comprehension with both: for-in & if-in... to "normal" python3 code


By : Pach
Date : March 29 2020, 07:55 AM
will be helpful for those in need please don't close this, since i'm a newbie to py3. Help me to "convert" from a list-comprehension with both: for-in & if-in... to "normal" python3 code with loops & conditions: , Nithin Varghese's answer is almost correct.
code :
friends = ["Wolf", "Frootie", "charlean", "Jenny"]
guests = ["xavier", "Bobbie", "wolf", "Charlean", "ashley"]

#friends_lcase = [f.lower() for f in friends]
friends_lcase = []
for f in friends:
    friends_lcase.append(f.lower()) <--- Change

# present_friends = [
#   name.title() for name in guests if name.lower() in friends_lcase
# ]

present_friends = []
for name in guests:
    if name.lower() in friends_lcase:
        present_friends.append(name.title())


print(present_friends)
Can "list comprehension" be considered as "functional programming"?

Can "list comprehension" be considered as "functional programming"?


By : Michael Rapp
Date : March 29 2020, 07:55 AM
hop of those help? Yes, it's definitely a functional technique, particularly assuming that all of those members are fields or pure functions. It's just syntactic sugar for 0 or more flatMaps followed by 1 map (with if clauses translated to withFilter).
Without the yield at the end, it acts more like the imperative for, translating to 1 or more foreachs; foreach typically being used for executing statements for their side effects.
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