Need help to understand this code

Question

I have a problem to understand this code

def get_device_sensors(device_id):
    return [elm["sensor"] for elm in db.DB.query("select * from data ",
                                                 (device_id,))]

I'm new in python. I don't know what does elm["sensor"] mean.
Does it mean?:

for elm in db.DB.query("select * from data ")
    return elm['sensor']

Answer 1

It means exactly the same as:

results = []
for elem in db.DB.query("SELECT * FROM data", (device_id,)):
    results.append(elem['sensor'])

return results

This is a list comprehension , like @khelwood noted, which means you are constructing a list from something you can iterate over.

To summarise:

[ <operation> for <values> in <iterable> ]

basically is a shorthand for:

accumulator = []
for <values> in <iterable>:
     accumulator.append(<operation>)
# accumulator contains the result now.

<operation> usually does something with <values> , but it may also be a constant or just the <values> itself.