Why do I get a KeyError in this generator?

Question

I've got the following dictionary: ``` d = { 'A': { 'param': { '1': { 'req': True, }, '2': { 'req': True, }, }, }, 'B': { 'param': { '3': { 'req': True, }, '4': { 'req': False, }, }, }, } ``` I want to have a generator which will give me for each first level keys, the required parameters. ``` req = {} for key in d: req[key] = (p for p in d[key]['param'] if d[key]['param'][p].get('req', False)) ``` So here, for each key in `d`, I get parameter `p` only if `req` is `True`. However, when I try to use my generator, it raises a `KeyError` exception: ``` >>> req {'A': at 0x27b8960>, 'B': at 0x27b8910>} >>> for elem in req['A']: ... print elem --------------------------------------------------------------------------- KeyError Traceback (most recent call last) in () ----> 1 for elem in req['A']: 2 print elem 3 in ((p,)) 1 for key in d: ----> 2 req[key] = (p f…

Accepted Answer

The generator expressions you assign to `req[key]` binds on the `key` variable. But `key` changes from 'A' to 'B' in the loop. When you iterate over the first generator expression, it will evaluate `key` to 'B' in its `if` condition, even though `key` was 'A' when you created it.

The conventional way to bind to a variable's value and not its reference, is to wrap the expression in a lambda with a default value, and then call it immediately.

```
for key in d:
    req[key] = (lambda key=key: (p for p in d[key]['param'] if d[key]['param'][p].get('req', False)))()
```

Result:

```
1
2
```

Why do I get a KeyError in this generator?

Solution

Problem