Define __iter__ and/or __contains__ for pybamm.ParameterValues

[ejfdickinson]

Description

The pybamm.ParameterValues is dict-like, and through its __getitem__ and __setitem__ methods allows dict-like key access. However, it does not support the in keyword, which for a dict will treat the keys as an iterable. This means that natural code like:

if "Negative particle radius [m]" in parameter_values:
    # do something

will not work (generally raises a KeyError). It is currently necessary to instead write explicitly:

if "Negative particle radius [m]" in parameter_values.keys():
    # do something

Defining the missing method __iter__ (and __contains__ if preferred) would allow in to work in logical test and for-loop contexts.

Motivation

Code is cleaner if pybamm.ParameterValues looks like a souped-up dict always, not just sometimes. Key-value accessors are expected to iterate over their keys and to allow logical test on the presence/absence of keys. PyBaMM-containing code looks 'asymmetric' compared to code working with actual dict objects due to the requirement to call into pybamm.ParameterValues.keys() specifically.

Was there any historic reason for the decision to exclude these methods?

Possible Implementation

No response

Additional context

No response

[rtimms]

AFAIK these weren't intentionally excluded.

[martinjrobins]

I just ran into this problem today, please do implement these, it would be very helpful! :)

[kratman]

Yeah I have wanted this as well, but never go around to adding anything