e4f4ea47eb lint: Catch use of [] or {} as default parameter values in Python functions (practicalswift)
25dd867150 Avoid using mutable default parameter values (practicalswift)
Pull request description:
Avoid common Python default parameter gotcha when mutable `dict`/`list`:s are used as default parameter values.
Examples of this gotcha caught during review:
* https://github.com/bitcoin/bitcoin/pull/16673#discussion_r317415261
* https://github.com/bitcoin/bitcoin/pull/14565#discussion_r241942304
Perhaps surprisingly this is how mutable list and dictionary default parameter values behave in Python:
```
>>> def f(i, j=[], k={}):
... j.append(i)
... k[i] = True
... return j, k
...
>>> f(1)
([1], {1: True})
>>> f(1)
([1, 1], {1: True})
>>> f(2)
([1, 1, 2], {1: True, 2: True})
```
In contrast to:
```
>>> def f(i, j=None, k=None):
... if j is None:
... j = []
... if k is None:
... k = {}
... j.append(i)
... k[i] = True
... return j, k
...
>>> f(1)
([1], {1: True})
>>> f(1)
([1], {1: True})
>>> f(2)
([2], {2: True})
```
The latter is typically the intended behaviour.
This PR fixes two instances of this and adds a check guarding against this gotcha going forward :-)
ACKs for top commit:
Sjors:
Oh Python... ACK e4f4ea47eb. Testing tip: swap the two commits.
Tree-SHA512: 56e14d24fc866211a20185c9fdb274ed046c3aed2dc0e07699e58b6f9fa3b79f6d0c880fb02d72b7fe5cc5eb7c0ff6da0ead33123344e1a872209370c2e49e3f
cc40b55da7