How to indent Python list-comprehensions?
List comprehensions can be useful in certain situations, but they can also be rather horrible to read.. As a slightly exaggerated example, how would you indent the following?
allUuids = [x.id for x in self.db.query(schema.allPostsUuid).execute(timeout = 20) if x.type == "post" and x.deleted is not False]
Asked by: Edgar503 | Posted: 28-01-2022
Answer 1
It depends on how long they are. I tend to structure them like so:
[x.id for x
in self.db.query(schema.allPostsUuid).execute(timeout=20)
if x.type == 'post'
and x.deleted is not False
and ...
and ...]
That way every expression has its own line.
If any line becomes too big I like to extract it out in a lambda or expression:
transform = lambda x: x.id
results = self.db.query(schema.allPostsUuid).execute(timeout=20)
condition = lambda x: x.deleted is not False and ... and ...
[transform(x) for x in results if condition(x)]
And then if a lambda becomes too long it gets promoted to a function.
Answered by: Gianna586 | Posted: 01-03-2022Answer 2
Where I work, our coding guidelines would have us do something like this:
all_posts_uuid_query = self.db.query(schema.allPostsUuid)
all_posts_uuid_list = all_posts_uuid_query.execute(timeout=20)
all_uuid_list = [
x.id
for x in all_posts_uuid_list
if (
x.type == "post"
and
not x.deleted # <-- if you don't care about NULLs / None
)
]
Answer 3
allUuids = [x.id
for x in self.db.query(schema.allPostsUuid).execute(timeout = 20)
if x.type == "post" and x.deleted is not False]
Answer 4
For me that's too much. Maybe it's just a terrible example, since "type" and "deleted" would clearly be part of the db query.
I tend to think that if a list comprehension spans multiple lines it probably shouldn't be a list comprehension. Having said that, I usually just split the thing at "if" like other people have and will answer here.
Answered by: Dainton993 | Posted: 01-03-2022Answer 5
You should not use a list comprehension for that.
List comprehensions are an awesome feature, but they are meant to be shortcuts, not regular code.
For such a long snippet, you should use ordinary blocs :
allUuids = []
for x in self.db.query(schema.allPostsUuid).execute(timeout = 20) :
if x.type == "post" and x.deleted is not False :
allUuids.append(x.id)
Exactly the same behavior, much more readable. Guido would be proud of you :-)
Answered by: Catherine961 | Posted: 01-03-2022Answer 6
If you're set on a comprehension orestis's answer is good.
For more complex comprehensions like that I'd suggest using a generator with yield:
allUuids = list(self.get_all_uuids())
def get_all_uuids(self):
for x in self.db.query(schema.allPostsUuid).execute(timeout = 20):
if x.type == "post" and x.deleted is not False:
yield x.id
Answer 7
How about:
allUuids = [x.id for x in self.db.query(schema.allPostsUuid).execute(timeout = 20)
if (x.type == "post" and x.deleted is not False)]
Generally, long lines can be avoided by pre-computing subexpressions into variables, which might add a minuscule performance cost:
query_ids = self.db.query(schema.allPostsUuid).execute(timeout = 20)
allUuids = [x.id for x in query_ids
if (x.type == "post" and x.deleted is not False)]
By the way, isn't 'is not False' kind-of superfluous ? Are you worried about differentiating between None and False ? Because otherwise, it suffices to leave the condition as only: if (x.type == "post" and x.deleted)
Answer 8
allUuids = [
x.id
for x in self.db.query(schema.allPostsUuid).execute(timeout = 20)
if x.type == "post"
and x.deleted
]
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