Python : Zip function

Question

t1 = ("a", "b", "c", "d")
t2 = (1, 2, 3, 4)
z = zip(t1, t2)
print(list(z))
print(dict(z))

It seems like we can cast the zip object only once. After z = zip(t1, t2) only the first casting either list(z) or dict(z) works and the other one does not work. Why is it so ?

[zip returns a generator in python3](https://docs.python.org/3/library/functions.html#zip) — Sohaib Farooqi, Mar 08 '18 at 01:21
To reiterate @bro-grammer, a generator instance that has been exhausted does not restart. You must redefine the generator, e.g. by feeding directly into `list`: `list(zip(t1, t2))`. — jpp, Mar 08 '18 at 01:23
@jpp please post it as answer, so that I can accept it as the solution — Akshay J, Mar 08 '18 at 01:43

score 0 · Answer 1 · answered Mar 08 '18 at 01:26

0

Another option is to use the itertools.tee() function to create a second version of your generator:

from itertools import tee
t1 = ("a", "b", "c", "d")
t2 = (1, 2, 3, 4)
z,z_backup = tee(zip(t1, t2))
print(list(z))
print(dict(z_backup))

answered Mar 08 '18 at 01:26

William Feirie

624
3
7

Beware, for simple tuples as sources this is fine, but if `t1` and `t2` are infinite, `tee`ing them will buffer all data consumed by only one of the tees, which could produce exploding memory consumption if you're not careful. In your example, after the next-to-last line executes, the entire `zip`ped data is buffered! – wberry Mar 08 '18 at 01:36
Note: If you'll end up running out one of the `tee`d generators to exhaustion before even beginning to iterate the other, you're better off constructing a `list`/`tuple` from the original generator and iterating it an arbitrary number of times. `tee` would have to store the same data as the `list`/`tuple`, but imposes additional overhead; the overhead is only worth it if you're advancing the `tee`-ed generators in parallel (or close to it) where the cache stays small. In this case, just doing `z = tuple(zip(t1, t2))`, then `print(list(z))`, `print(dict(z))` makes more sense. – ShadowRanger Mar 08 '18 at 01:38

Python : Zip function

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