Python multilevel getattr with string

Question

I have a function (here called analyze_data) that takes an object and a string as input, and uses the string to "extract" data via getattr. This works fine when the data are in the "frist" level of attributes (data1). However, I don't know what I can pass to the function to access the data when they are in a lower level (data2). Passing a dotted string (as shown below) does not work.

class a:
    pass

instance1 = a()

# Create 1st data set
instance1.data1 = [1,2,3]

# Create 2nd data set
instance1.subdata = a()
instance1.subdata.data2 = [4,5,6]


def analyze_data(myvar, dataname):
    data = getattr(myvar, dataname)
    # Do something with data, e.g.:
    print(str(data[0]))
    
analyze_data(instance1, 'data1')
analyze_data(instance1, 'subdata.data2')

What is the best way to access data2 without changing the existing function analyze_data too much?

gold_cy · Accepted Answer · 2022-01-12T12:40:46.880

Instead of using getattr you should use attrgetter which allows you to specify using the dot notation. This does require a minor change to your function as shown below.

from operator import attrgetter

def analyze_data(myvar, dataname):
    fn = attrgetter(dataname)
    data = fn(myvar)
    # Do something with data, e.g.:
    print(str(data[0]))

analyze_data(instance1, 'data1')
1

analyze_data(instance1, 'subdata.data2')
4

score 1 · Answer 2 · answered Jan 12 '22 at 12:36

1

functools.reduce is another option.

>>> from functools import reduce
>>> reduce(getattr, 'subdata.data2'.split('.'), instance1)
[4, 5, 6]

answered Jan 12 '22 at 12:36

timgeb

76,762
20
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145

score 1 · Answer 3 · answered Jan 12 '22 at 12:36

1

One way to achieve what you want without changing the function at all is this:

class a:
pass

instance1 = a()

# Create 1st data set
instance1.data1 = [1,2,3]

# Create 2nd data set
instance1.subdata = a()
instance1.subdata.data2 = [4,5,6]


def analyze_data(myvar, dataname):
    data = getattr(myvar, dataname)
    # Do something with data, e.g.:
    print(str(data[0]))

analyze_data(instance1, 'data1')
analyze_data(instance1.subdata, 'data2')

answered Jan 12 '22 at 12:36

It works on my pc

166
7

Very good answer! The only reason I did not do it this way is because in reality my `analyze_data` is more complex and does not take `instance1` directly as in argument, but a list of instances. – Dave Jan 13 '22 at 09:57

score 0 · Answer 4 · answered Jan 12 '22 at 12:35

0

You can recurse into the data by splitting the dotted path and following it:

def analyze_data(myvar, dataname):
    for dn in dataname.split('.'):
        myvar = getattr(myvar, dn)
    # Do something with data, e.g.:
    print(str(myvar[0]))

This keeps updating myvar until you are at the end of the path.

answered Jan 12 '22 at 12:35

vaizki

1,678
1
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2

The answer does not deserve to be downvoted. It explicitly demonstartes the logic of the solution. That's how `attrgetter` does the job. – Jan 12 '22 at 13:01
1

I tried to follow the brief and not modify the function "too much" instead of using a library or clever one-liner to obfuscate it. – vaizki Jan 12 '22 at 13:04

Python multilevel getattr with string

4 Answers4