[issue33098] add implicit conversion for random.choice() on a dict

[Aristide Grange]

Aristide Grange <aristide.gra...@gmail.com> added the comment:

My bad... For my reference to Python 2, I relied on my memory only, which 
starts to vanish. Really sorry about that. Yes, `random.choice(d)` (mostly) 
fails in Python 2 too, with an error message that I better understand after 
reading your explanation.

So, in Python 2/3, when `random.choice()` is applied to a dictionary, it draws 
a random integer i in [0, len(d)[ and tries to return the _value_ `d[i]`. It's 
quite unexpected, for me at last. According to the doc:

random.choice(seq)
Return a random element from the non-empty sequence seq. If seq is empty, 
raises IndexError.

In Python 3, evaluating `choice(d.keys())` raises "TypeError: 'dict_keys' 
object does not support indexing". Shouldn't `choice(d)` _always_ fail with the 
same error message? I am not sure to see any legitimate use for the current 
behavior.

With regard to the repeated refusal to hide the fact that `choice`-ing among 
the keys of a dictionary is a linear operation, I can understand this decision. 
The general interest does not necessary align with that of an algorithmic 
teacher which only uses Python as a support language for introducing students 
to basic / transversal datatypes such as lists, arrays, dictionaries, sets, and 
prefers to avoid speaking of `dict_keys` and other Python's niceties...

Anyway, thanks a lot for your detailed and patient answer.

--

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[issue33098] add implicit conversion for random.choice() on a dict

[Aristide Grange]

New submission from Aristide Grange <aristide.gra...@gmail.com>:

In Python 3, the expression:

```python
random.choice(d)
```

where `d` is a `dict`, raises this error:

```
~/anaconda3/lib/python3.6/random.py in choice(self, seq)
256 except ValueError:
257 raise IndexError('Cannot choose from an empty sequence') 
from None
--> 258 return seq[i]
259 
260 def shuffle(self, x, random=None):

KeyError: 2
```

Converting `d` into a list restores the Python 2's behavior:

```python
random.choice(list(d))
```

I am aware that the keys of a dict have now their own type. But IMHO the error 
message is rather uninformative, and above all, couldn't this conversion be 
made implicitely under the hood?

--
messages: 314062
nosy: Aristide Grange
priority: normal
severity: normal
status: open
title: add implicit conversion for random.choice() on a dict
type: enhancement
versions: Python 3.4

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