# Python: Be cautious when using generators/iterators in nested loops

Simply because generators/itertors would drain.

```
X = [1,2]
Y = [3,4]
[(x, y) for x in X for y in Y] # OK
# Output: [(1, 3), (1, 4), (2, 3), (2, 4)]
```

```
X = iter([1,2])
Y = [3,4]
[(x, y) for x in X for y in Y] # OK
# Output: [(1, 3), (1, 4), (2, 3), (2, 4)]
```

```
X = iter([1,2])
Y = iter([3,4])
[(x, y) for x in X for y in Y] # What the hell!
# Output: [(1, 3), (1, 4)]
```

Otherwise you can create a new generator/iterator for each run.

```
X = iter([1,2])
# Y = iter([3,4])
[(x, y) for x in X for y in iter([3,4])] # OK
# Output: [(1, 3), (1, 4), (2, 3), (2, 4)]
```

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