What does for row_number, row in enumerate(cursor):
do in Python?
What does enumerate
mean in this context?
The enumerate()
function adds a counter to an iterable.
So for each element in cursor
, a tuple is produced with (counter, element)
; the for
loop binds that to row_number
and row
, respectively.
Demo:
>>> elements = ('foo', 'bar', 'baz')
>>> for elem in elements:
... print elem
...
foo
bar
baz
>>> for count, elem in enumerate(elements):
... print count, elem
...
0 foo
1 bar
2 baz
By default, enumerate()
starts counting at 0
but if you give it a second integer argument, it'll start from that number instead:
>>> for count, elem in enumerate(elements, 42):
... print count, elem
...
42 foo
43 bar
44 baz
If you were to re-implement enumerate()
in Python, here are two ways of achieving that; one using itertools.count()
to do the counting, the other manually counting in a generator function:
from itertools import count
def enumerate(it, start=0):
# return an iterator that adds a counter to each element of it
return zip(count(start), it)
and
def enumerate(it, start=0):
count = start
for elem in it:
yield (count, elem)
count += 1
The actual implementation in C is closer to the latter, with optimisations to reuse a single tuple object for the common for i, ...
unpacking case and using a standard C integer value for the counter until the counter becomes too large to avoid using a Python integer object (which is unbounded).