Create my own method for DataFrames (python)

问题

So I wanted to create a module for my own projects and wanted to use methods. For example I wanted to do:

from mymodule import *
df = pd.DataFrame(np.random.randn(4,4))
df.mymethod()

Thing is it seems I can't use .myfunc() since I think I can only use methods for the classes I've created. A work around is making mymethod a function and making it use pandas.Dataframes as a variable:

myfunc(df)

I don't really want to do this, is there anyway to implement the first one?

回答1:

If you really need to add a method to a pandas.DataFrame you can inherit from it. Something like:

mymodule:

import pandas as pd

class MyDataFrame(pd.DataFrame):
    def mymethod(self):
        """Do my stuff"""

Use mymodule:

from mymodule import *
df = MyDataFrame(np.random.randn(4,4))
df.mymethod()

To preserve your custom dataframe class:

pandas routinely returns new dataframes when performing operations on dataframes. So to preserve your dataframe class, you need to have pandas return your class when performing operations on an instance of your class. That can be done by providing a _constructor property like:

class MyDataFrame(pd.DataFrame):

    @property
    def _constructor(self):
        return MyDataFrame

    def mymethod(self):
        """Do my stuff"""

Test Code:

class MyDataFrame(pd.DataFrame):

    @property
    def _constructor(self):
        return MyDataFrame

df = MyDataFrame([1])
print(type(df))
df = df.rename(columns={})
print(type(df))

Test Results:

<class '__main__.MyDataFrame'>
<class '__main__.MyDataFrame'>

回答2:

Nice solution can be found in ffn package. What authors do:

from pandas.core.base import PandasObject
def your_fun(df):
    ...
PandasObject.your_fun = your_fun

After that your manual function "your_fun" becomes a method of pandas.DataFrame object and you can do something like

df.your_fun()

This method will be able to work with both DataFrame and Series objects

回答3:

This topic is well documented as of Nov 2019: Extending pandas

Note that the most obvious technique - Ivan Mishalkin's monkey patching - was actually removed at some point in the official documentation... probably for good reason.

Monkey patching works fine for small projects, but there is a serious drawback for a large scale project: IDEs like Pycharm can't introspect the patched-in methods. So if one right clicks "Go to declaration", Pycharm simple says "cannot find declaration to go to". It gets old fast if you're an IDE junkie.

I confirmed that Pycharm CAN introspect both the "custom accessors" and "subclassing" methods discussed in the official documentation.

来源：https://stackoverflow.com/questions/43504068/create-my-own-method-for-dataframes-python

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python

pandas

methods

module