Calling a class function inside of init

Question

I'm writing some code that takes a filename, opens the file, and parses out some data. I'd like to do this in a class. The following code works:

class MyClass():
    def __init__(self, filename):
        self.filename = filename 

        self.stat1 = None
        self.stat2 = None
        self.stat3 = None
        self.stat4 = None
        self.stat5 = None

        def parse_file():
            #do some parsing
            self.stat1 = result_from_parse1
            self.stat2 = result_from_parse2
            self.stat3 = result_from_parse3
            self.stat4 = result_from_parse4
            self.stat5 = result_from_parse5

        parse_file()

But it involves me putting all of the parsing machinery in the scope of the __init__ function for my class. That looks fine now for this simplified code, but the function parse_file has quite a few levels of indention as well. I'd prefer to define the function parse_file() as a class function like below:

class MyClass():
    def __init__(self, filename):
        self.filename = filename 

        self.stat1 = None
        self.stat2 = None
        self.stat3 = None
        self.stat4 = None
        self.stat5 = None
        parse_file()

    def parse_file():
        #do some parsing
        self.stat1 = result_from_parse1
        self.stat2 = result_from_parse2
        self.stat3 = result_from_parse3
        self.stat4 = result_from_parse4
        self.stat5 = result_from_parse5

Of course this code doesn't work because the function parse_file() is not within the scope of the __init__ function. Is there a way to call a class function from within __init__ of that class? Or am I thinking about this the wrong way?

Is there any reason the code example needs five versions of "stat"? It would make it easier to read if there would be only one. — 465b, Apr 29 '20 at 15:23

Lewis Diamond · Accepted Answer · 2021-07-14T23:43:27.253

299

Call the function in this way:

self.parse_file()

You also need to define your parse_file() function like this:

def parse_file(self):

The parse_file method has to be bound to an object upon calling it (because it's not a static method). This is done by calling the function on an instance of the object, in your case the instance is self.

edited Jul 14 '21 at 23:43

answered Sep 28 '12 at 19:45

Lewis Diamond

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score 60 · Answer 2 · edited Sep 28 '12 at 20:05

60

If I'm not wrong, both functions are part of your class, you should use it like this:

class MyClass():
    def __init__(self, filename):
        self.filename = filename 

        self.stat1 = None
        self.stat2 = None
        self.stat3 = None
        self.stat4 = None
        self.stat5 = None
        self.parse_file()

    def parse_file(self):
        #do some parsing
        self.stat1 = result_from_parse1
        self.stat2 = result_from_parse2
        self.stat3 = result_from_parse3
        self.stat4 = result_from_parse4
        self.stat5 = result_from_parse5

replace your line:

parse_file()

with:

self.parse_file()

edited Sep 28 '12 at 20:05

Paolo Moretti

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answered Sep 28 '12 at 19:45

Paritosh Singh

6,288
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Could you explain why it must be used `self.parse_file()` and not `parse_file()`? – ivanleoncz Aug 08 '17 at 04:32
@paritoshsingh should `def parse_file(self):` not be nested under the `__init__` function so that you don't have a partially initialised object? – ron_g Apr 23 '18 at 19:19
@ivanleoncz As parse_file is instance method, we need to use reference object to call same. – Paritosh Singh May 06 '18 at 02:14
@rong If this code is venerable and you don't want to allow setting parse file from outside, yes it should be nested. – Paritosh Singh May 06 '18 at 02:24
Consider using static methods https://www.programiz.com/python-programming/methods/built-in/staticmethod – Sandesh Ghanta Dec 29 '20 at 13:35

score 18 · Answer 3 · answered Sep 28 '12 at 19:45

18

How about:

class MyClass(object):
    def __init__(self, filename):
        self.filename = filename 
        self.stats = parse_file(filename)

def parse_file(filename):
    #do some parsing
    return results_from_parse

By the way, if you have variables named stat1, stat2, etc., the situation is begging for a tuple: stats = (...).

So let parse_file return a tuple, and store the tuple in self.stats.

Then, for example, you can access what used to be called stat3 with self.stats[2].

answered Sep 28 '12 at 19:45

unutbu

842,883
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I agree, I just put the self.stat1 through self.stat5 in there to show that there were some class variables that I was assigning to. In the actual code I have a more elegant solution. – PythonJin Sep 28 '12 at 19:58
How should this be altered if I want to the `parse_file` function to be a method of the `MyClass` object. Where would `self` be necessary? – n1k31t4 Jul 01 '18 at 13:25

score 1 · Answer 4 · answered Sep 28 '12 at 19:47

You must declare parse_file like this; def parse_file(self). The "self" parameter is a hidden parameter in most languages, but not in python. You must add it to the definition of all that methods that belong to a class. Then you can call the function from any method inside the class using self.parse_file

your final program is going to look like this:

class MyClass():
  def __init__(self, filename):
      self.filename = filename 

      self.stat1 = None
      self.stat2 = None
      self.stat3 = None
      self.stat4 = None
      self.stat5 = None
      self.parse_file()

  def parse_file(self):
      #do some parsing
      self.stat1 = result_from_parse1
      self.stat2 = result_from_parse2
      self.stat3 = result_from_parse3
      self.stat4 = result_from_parse4
      self.stat5 = result_from_parse5

score 0 · Answer 5 · answered Sep 28 '12 at 19:47

0

In parse_file, take the self argument (just like in __init__). If there's any other context you need then just pass it as additional arguments as usual.

answered Sep 28 '12 at 19:47

Teo Klestrup Röijezon

5,097
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score -15 · Answer 6 · answered Sep 28 '12 at 19:44

-15

I think that your problem is actually with not correctly indenting init function.It should be like this

class MyClass():
     def __init__(self, filename):
          pass

     def parse_file():
          pass

answered Sep 28 '12 at 19:44

Zed

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1

Neither is your code. Unless you put the `@staticmethod` decorator on top of `parse_file` – rantanplan Sep 28 '12 at 19:46
at least add the missing `self` in your `parse_file` method. – rantanplan Sep 28 '12 at 19:49
Oops, sorry there should have been an extra layer of indenting under the "class MyClass():" line. I've fixed it above, but the question remains the same. – PythonJin Sep 28 '12 at 19:55

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