confusion about classes, objects and methods python

Question

I am trying to figure out why 'v3' below doesn't work and raises an attribute error. What makes v1 and v2 work and not v3? The code is short and reproducible, and I think it is also simple enough to not need explanation but pls let me know if anything is not clear.

import numpy as np
import pandas as pd

class Example(object):
    def __init__(self, ts_df):
        self.all_df = ts_df

    def simple_av(self, lookback=""):
        self.agg = self.all_df.mean(axis=1)

class Example_two(object):
    def __init__(self, ts_df, method):
        self.ts = ts_df
        self.method = method

    def apply_method(self, **kwargs):
        self.output = self.method(self.ts, **kwargs)

ts = pd.DataFrame(np.random.rand(100,2))

'''v1'''
ex = Example(ts)
ex.simple_av()
print (ex.agg.head())

'''v2'''
func = pd.rolling_mean
ex = Example_two( ts, func)
req_args = dict({'window': 3})
ex.apply_method(**req_args)
print (ex.output.head())

'''v3'''
func = Example.simple_av
ex= Example(ts)
ex.func()

Jean-François Fabre · Accepted Answer · 2017-12-12 22:36:50Z

2

The third example fails because func exists but ex.func does not. ex.func looks up "func" in the attributes of Example class and fails.

You could call func in procedural style, passing ex as first argument:

func(ex)

which is equivalent to:

ex.simple_av()

(the 2 first examples use a call to a defined method, so it works)

answered Dec 12, 2017 at 22:36

Jean-François Fabre♦

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