implement default class instance method

Question

i have the following class which i use when declaring 'constants' to map one value to another. Is there away in python implement a default method such that anytime an instance of this class referenced without a specific method, a default method is executed?

class FieldMap:
    def __init__(self, old_field, new_field):
        self._fld_map = (old_field, new_field)

    def old_fld(self):
        return self._fld_map[0]

    def new_fld(self):
        return self._fld_map[1]

SEC_ID = FieldMap('value','price')
SEC_NAME = FieldMap('entity_name', 'security_name')
SEC_TICKER = FieldMap('entity_attr', 'ticker')

#Edit: updating example to provide a real example of what i want to achieve
dict = {}
dict['price'] = 1234
print dict[SEC_ID]  <--would like this to print out the value 1234 because ideally, the default method would call new_fld() and return 'price'

chepner · Accepted Answer · 2014-09-29 21:23:52Z

1

It doesn't call a default method, but it sounds like you just want to override __str__:

def __str__(self):
    return self._fld_map[1]

Note that this is just the definition of new_fld, so you could simply add the following to your class:

__str__ = new_fld

answered Sep 29, 2014 at 21:23

chepner

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2 Comments

mike01010 Over a year ago

thanks, that works for print. it doesn't work for say something like this though mydict[SEC_ID] = 1234. i know that isn't exactly what i asked, maybe i will update the question

Platinum Azure Over a year ago

In that case, you need to either call str(SEC_ID) or store elements using dict[SEC_ID] = 1234 and then dict[SEC_ID] to retrieve. In general, you can't expect Python to magically convert types-- how would it know to convert to string or integer in most cases?

Platinum Azure · Accepted Answer · 2014-09-29 21:30:25Z

1

There's no way to implement an automatic method call in all cases, but it is possible to hook into certain method calls. In the case of print, Python will attempt to call a __str__ method on the instance if one exists. So you could do something like this:

class FieldMap:
    def __init__(self, old_field, new_field):
        self._fld_map = (old_field, new_field)

    def old_fld(self):
        return self._fld_map[0]

    def new_fld(self):
        return self._fld_map[1]

    def __str__(self):
        return self.new_fld()

edited Sep 29, 2014 at 21:30

answered Sep 29, 2014 at 21:23

Platinum Azure

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