Why does my Python class claim that I have 2 arguments instead of 1?

Question

#! /usr/bin/env python
import os
import stat
import sys
class chkup:

        def set(file):
                filepermission = os.stat(file)
                user_read()
                user_write()
                user_exec()

        def user_read():
                """Return True if 'file' is readable by user 
            """
            # Extract the permissions bits from the file's (or
            # directory's) stat info.
                b = bool(filepermission.st_mode & stat.S_IRUSR)
                print b
            return b

        def user_write():
                """Return True if 'file' is readable by user 
            """
            # Extract the permissions bits from the file's (or
            # directory's) stat info.
                b = bool(filepermission.st_mode & stat.S_WRUSR)
                print b
            return b

        def user_exec():
                """Return True if 'file' is readable by user 
            """
            # Extract the permissions bits from the file's (or
            # directory's) stat info.
                b = bool(filepermission.st_mode & stat.S_IXUSR)
                print b
            return b

def main():
        i = chkup()
        place = '/net/home/f08/itsrsw1/ScriptingWork/quotacheck'
        i.set(place)

if __name__ == '__main__':
        main()

With that code I receive

> Traceback (most recent call last):
  File "chkup.py", line 46, in <module>
    main()
  File "chkup.py", line 43, in main
    i.set(place)
TypeError: set() takes exactly 1 argument (2 given)

Any thoughts?

Answer 1

The first argument for a python class method is the self variable. If you call classInstance.method(parameter), the method is invoked as method(self, parameter).

So, when you're defining your class, do something like this:

class MyClass(Object): 
    def my_method(self, parameter): 
        print parameter

You might want to read through the Python tutorial.

Answer 2

Because you're not passing the object (generally referred to as self) as the first parameter to your methods. In Python, a call like this:

my_obj.do_something(my_other_obj)

is essentially desugared into a call like this:

MyClass.do_something(my_obj, my_other_obj)

Thus, Python is looking for a method signature like this:

class MyClass(object):
    def do_something(self, my_other_obj):
        self.my_var = my_other_obj

So you should pass the object (generally called self) as the first parameter to a method.

Answer 3

You need to explicitly pass self variable, which represents an instance of a class, e.g.:

def set(self, file):
    filepermission = os.stat(file)
    self.user_read()
    self.user_write()
    self.user_exec()

It doesn't have to be called self but it's a good convention to follow, and your code will be understood by other programmers.