OOP Variable Not Defined in Python Class -Need Help Understanding init

Question

I'm very new to OOP and I have been trying to write a class I can import which will help me with parsing files. I realize I do not need to make a class to do this, but thought I'd try to so I can start getting familiar with OOP.

This code works

import re
import os

destdir = r"FilePathToDirectory"

class Parsey():                            

    def GetNums(self,source, destination, trim = True):
        with open (os.path.join(destdir,source), 'r') as infile:
            with open (os.path.join(destdir,destination), 'w') as outfile:
                for line in infile:
                    #Look for number patern match
                    if re.match(r'(.*)\d\d-\d\d-\d\d\d\d(.*)', line):
                        #If trim is True clean up the line
                        if trim == True:
                            #Find the first numeric character
                            firstdig = re.search("\d",line)
                            #Set the firstdig variable to the integer of that index
                            firstdig = firstdig.start()
                            #Set the line equal to only the begining and ending indexes of the number
                            line=line[firstdig:firstdig+10]
                            #Remove the dashes from the string
                            line = line.replace('-','')
                            outfile.writelines(line+'\n')
                        else:
                            outfile.writelines(line)

This code does not and I'm not sure why it doesn't.

import re
import os

class Parsey():

    def __init__(self, destdir=''):
        self.destdir = r"FilePathToDirectory"

    def GetNums(self,source, destination, trim = True):
        with open (os.path.join(destdir,source), 'r') as infile:
            with open (os.path.join(destdir,destination), 'w') as outfile:
                for line in infile:
                    #Look for number patern match
                    if re.match(r'(.*)\d\d-\d\d-\d\d\d\d(.*)', line):
                        #If trim is True clean up the line
                        if trim == True:
                            #Find the first numeric character
                            firstdig = re.search("\d",line)
                            #Set the firstdig variable to the integer of that index
                            firstdig = firstdig.start()
                            #Set the line equal to only the begining and ending indexes of the number
                            line=line[firstdig:firstdig+11]
                            #Remove the dashes from the string
                            line = line.replace('-','')
                            outfile.writelines(line+'\n')
                        else:
                            outfile.writelines(line)

I receive the error: line 10, in GetNums with open (os.path.join(destdir,source), 'r') as infile: NameError: name 'destdir' is not defined

It was my understanding that the namespace of the class object would allow the functions within the class to see all variables declared in that class.

self.destdir. Also, you are allowing for destdir to be passed to the initializer, but never using it? — Andrew Li
– Andrew Li, Commented Aug 3, 2018 at 20:26

Mike Müller · Accepted Answer · 2018-08-03 20:51:54Z

1

You need to change line 10 to:

with open (os.path.join(self.destdir, destination), 'w') as outfile:

In your case Python looks for testdir inside GetNums first and, if it cannot find it there, it will look for this name in the module. It does not magically use tesdir from __init__. The name self stands for the instance you will create later. So in __init__ you essentially set mysinstance.testdir and later in GetNums you can access with mysinstance.testdir. self is just the placeholder for mysinstance, i.e the instance you create later.

You can read the detail in the documentation.

edited Aug 3, 2018 at 20:51

answered Aug 3, 2018 at 20:37

Mike Müller

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3 Comments

Michael Over a year ago

Thank you much! That did the trick. So it was that the function looked for a global variable. Couldn't find it. Looked for a local variable. Couldn't find it. But it does not look specifically in the object itself unless explicitly told to do so by the .self. Correct me if I'm wrong. A link to good documentation explaining how this works if anyone knows any would be great.

Mike Müller Over a year ago

Added a link to the tutorial. This should give you enough background information.

Michael Over a year ago

Thanks again. This looks better than the one I was looking at on programiz.

Michael · Accepted Answer · 2018-08-03 20:47:03Z

@Mike Müller nailed it, but here is the corrected code in its entirety.

import re
import os

class Parsey():

    def __init__(self, destdir=''):
        self.destdir = r"FilePathToDirectory"

    def GetNums(self,source, destination, trim = True):
        with open (os.path.join(self.destdir,source), 'r') as infile:
            with open (os.path.join(self.destdir,destination), 'w') as outfile:
                for line in infile:
                    #Look for number patern match
                    if re.match(r'(.*)\d\d-\d\d-\d\d\d\d(.*)', line):
                        #If trim is True clean up the line
                        if trim == True:
                            #Find the first numeric character
                            firstdig = re.search("\d",line)
                            #Set the firstdig variable to the integer of that index
                            firstdig = firstdig.start()
                            #Set the line equal to only the begining and ending indexes of the number
                            line=line[firstdig:firstdig+10]
                            #Remove the dashes from the string
                            line = line.replace('-','')
                            outfile.writelines(line+'\n')
                        else:
                            outfile.writelines(line)

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