How to filter a list of lists based on a variable set of conditions with Python?

Question

Question related to this post.

I need help to filter a list of lists depending on a variable set of conditions.

Here is an extract of the list:

ex = [
    # ["ref", "type", "date"]
    [1, 'CB', '2017-12-11'],
    [2, 'CB', '2017-12-01'],
    [3, 'RET', '2017-11-08'],
    [1, 'CB', '2017-11-08'],
    [5, 'RET', '2017-10-10'],
]

I want to apply a final treatment like list(filter(makeCondition, ex)) where makeCondition(myList, **kwargs) function returns a boolean that enables to filter the list.

I have troubles building this function as adviced in this post, as the number of conditions defined in the **kwargs dictionnary is variable.

Examples of conditions sets:

conditions = {"ref": 3}
conditions = {"ref": 1, "type": "CB"}

Here is a start:

def makeConditions(myList, **p):

    # For each key:value in the dictionnary
    for key, value in p.items():

        if key == "ref":
            lambda x: x[0] == value
        elif key == "type":
            lambda x: x[1] == value
        elif key == "date":
            lambda x: x[2] == value

    # return chained conditions...

list(filter(makeConditions, ex))

I don't get the logical idea that various comments attempted to give in the post mentioned above... Do I have to execute the filter() function for each sublist or is it possible to do it for the whole global list? Any advices will be very welcome!

Answer 1

I would simply make a function that returns the conditional:

def makeConditions(**p):
    fieldname = {"ref": 0, "type": 1, "date": 2 }
    def filterfunc(elt):
        for k, v in p.items():
            if elt[fieldname[k]] != v: # if one condition is not met: false
                return False
        return True
    return filterfunc

Then you can use it that way:

>>> list(filter(makeConditions(ref=1), ex))
[[1, 'CB', '2017-12-11'], [1, 'CB', '2017-11-08']]
>>> list(filter(makeConditions(type='CB'), ex))
[[1, 'CB', '2017-12-11'], [2, 'CB', '2017-12-01'], [1, 'CB', '2017-11-08']]
>>> list(filter(makeConditions(type='CB', ref=2), ex))
[[2, 'CB', '2017-12-01']]