How to dynamically assign values to class properties in Python?

Question

I'm trying to do the following: After performing a regex group search, I'm trying to assign the results to the class properties by a specific order. the number of results from the regex search varies from 1-5 values.

class Classification():
    def __init__(self, Entry):
        self.Entry = Entry
        self.Section = ''
        self.Class = 'Null'
        self.Subclass = 'Null'
        self.Group = 'Null'
        self.Subgroup = 'Null'


    def ParseSymbol(self,regex):

        Properties_Pointers = [self.Section,self.Class,self.Subclass,self.Group,self.Subgroup]

        Pattern_groups = re.search(regex, self.Symbol)

        i = 0
        for group in Pattern_groups.groups():
            Properties_Pointers[i] = group
            i += 1

the problem is that for each loop iteration, instead of the class property, Properties_Pointers[i] gets the property's value (and of course in this case I can't assign the desired value to the property).

thanks.

Martijn Pieters · Accepted Answer · 2013-10-03 08:02:57Z

4

Refer to attribute names instead, and use the setattr() function to store a new value on self:

def ParseSymbol(self, regex):
    attributes = ['Section', 'Class', 'Subclass', 'Group', 'Subgroup']

    Pattern_groups = re.search(regex, self.Symbol)

    for group, attr in zip(Pattern_groups.groups(), attributes):
        setattr(self, attr, group)

setattr() lets you set attributes based on a variable, here taking from attributes; there is also a companion getattr() function to retrieve attributes dynamically.

edited Oct 3, 2013 at 8:02

answered Oct 3, 2013 at 7:34

Martijn Pieters

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Comments

space · Accepted Answer · 2013-10-03 07:57:04Z

1

setattr() will set the attributes of an object based on a string name. You can rewrite ParseSymbol above:

    def ParseSymbol(self,regex):

        Properties_Pointers = ['Section','Class','Subclass','Group','Subgroup']

        Pattern_groups = re.search(regex, self.Symbol)

        i = 0
        for group in Pattern_groups.groups():
            setattr(self, Properties_Pointers[i], group)
            i += 1

As a side note, you can iterate over both Pattern_groups.groups() and Pattern_Pointers simultaneously by using zip(). This cleans up the code by removing the index variable i and its incrementation:

        for pointer, group in zip(Properties_Pointers, Pattern_groups.groups()):
            setattr(self, pointer, group)

answered Oct 3, 2013 at 7:57

space

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2 Comments

Martijn Pieters Over a year ago

My original version did that, until I realized that there could be fewer groups than there are attributes.

Martijn Pieters Over a year ago

But then again, zip() will terminate when the shortest list is exhausted anyway. I need more caffeine, clearly.

dorian · Accepted Answer · 2013-10-03 07:38:26Z

0

If you know that your regex will always contain the same number of groups, you can just use tuple unpacking:

self.Section, self.Class, self.Subclass,self.Group, self.Subgroup = Pattern_groups.groups()

answered Oct 3, 2013 at 7:38

dorian

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7 Comments

Martijn Pieters Over a year ago

Except that the regular expression may contain fewer groups than attributes?

dorian Over a year ago

Yes, but in that case the original code would also assign the matching groups to the wrong attributes so I assume that all the groups will match for the expected input.

Martijn Pieters Over a year ago

Why would it do that? If the regex has 3 groups, then only the first three attributes are assigned to.

dorian Over a year ago

I was talking about the case where some groups might not match, e.g. r'(SECTION:.*?)?(CLASS:.*?)?(SUBCLASS:.*?)?'.But if there are less actual groups in the regex than there are properties, then of course tuple unpacking won't work.

Martijn Pieters Over a year ago

Clearly the original code did not use named groups; it was expecting to work with positional groups only. :-)

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