Python嵌套元组列表

Question

I have a mysql query that runs and selects all of the Id's that match the select statement:

first_seen_select = "SELECT Id FROM domains_archive WHERE current_epoch = " + current_epoch + " AND first_seen IS NULL"
cur.execute(first_seen_select)

The output of cur.fetchall() is

((1,), (2,), (3,), (4,), (5,), (6,), (7,))

How do i extract these nested tuple Id #'s and convert them into a single list that i can iterate over?

If i run the following i get:

>>> bleh = cur.fetchall()
>>> for i in bleh:
...   print(i)
... 
(1,)
(2,)
(3,)
(4,)
(5,)
(6,)
(7,)

Answer 1

you can use a simple list comprehension:

[y for x in l for y in x]

Or with more meaningful variable names:

[item for sublist in l for item in sublist]

this will result in:

In [8]: l = ((1,), (2,), (3,), (4,), (5,), (6,), (7,))

In [9]: [y for x in l for y in x]
Out[9]: [1, 2, 3, 4, 5, 6, 7]

Answer 2

You could use:

result = [x[0] for x in cur.fetchall()]
print(result)

Output

[1, 2, 3, 4, 5, 6, 7]

Answer 3

lt = ((1,), (2,), (3,), (4,), (5,), (6,), (7,))

from itertools import chain

>>> list(chain.from_iterable(lt))

[1, 2, 3, 4, 5, 6, 7]

Answer 4

There's a fun (although not clear/unpractical) way to flatten it using sum() :

In [1]: bleh = ((1,), (2,), (3,), (4,), (5,), (6,), (7,))

In [2]: sum(bleh, ())
Out[2]: (1, 2, 3, 4, 5, 6, 7)

It works like that - iterates over the outer tuple and sums all the values inside:

In [3]: (1,) + (2,) + (3,) + (4,) + (5,) + (6,) + (7,)
Out[3]: (1, 2, 3, 4, 5, 6, 7)

And, as you see, if you add a tuple to a tuple, you get another tuple with all the values of those tuples.

By passing an empty tuple - () - as a second argument to the sum() function, we basically tell it to use it as an initial argument to start adding from, so it would actually work like that:

In [4]: () + (1,) + (2,) + (3,) + (4,) + (5,) + (6,) + (7,)
Out[4]: (1, 2, 3, 4, 5, 6, 7)

Answer 5

A general way to create a list from a nested tuple would be:

def tupleToList(nestedTuple):
    def gen(tuple_):
        for item in tuple_:
            if type(item) is tuple:
                yield from gen(item)
            else:
                yield item
    return [value for value in gen(nestedTuple)]

Note that everything is flattened. Example usage:

>>> print(tupleToList((('a','b'), ('c', 'd', ('e')))))
['a', 'b', 'c', 'd', 'e']