Accessing class file from multiple methods in Python

Question

My question relates mostly to how you use the with keyword in a class in Python.

If you have a Class which contains a file object, how do you use the with statement, if at all.

For example, I don't use with here:

class CSVLogger:
    def __init__(self, rec_queue, filename):
        self.rec_queue = rec_queue
        ## Filename specifications
        self.__file_string__ = filename
        f = open(self.__file_string__, 'wb')
        self.csv_writer = csv.writer(f,  newline='', lineterminator='\n', dialect='excel')

If I then do things to the file in another method, For example:

    def write_something(self, msg):
        self.csv_writer(msg)

Is this suitable? Should I include the with somewhere? I'm just afraid that one the __init__ exits, the with exits and might close the file?

Never give your own attributes or methods double-underscores on both sides. Those names are intended for Python's own attributes, which you may override, but not define your own. — Daniel Roseman, Jul 01 '15 at 09:13

score 1 · Accepted Answer · answered Jul 01 '15 at 09:18

Yes, you are correct with automatically closes the file when its scope ends, so if you use with statement in your __init__() function, the write_something function would not work.

Maybe you can use the with statement in the main part of the program, and instead of opening the file in __init__() function you can pass in the file object as a parameter to the __init__() function. and then do all operations you would like to do in the file within the with block.

Example -

Class would look like -

class CSVLogger:
    def __init__(self, rec_queue, filename, f):
        self.rec_queue = rec_queue
        ## Filename specifications
        self.__file_string__ = filename
        self.csv_writer = csv.writer(f,  newline='', lineterminator='\n', dialect='excel')
    def write_something(self, msg):
        self.csv_writer(msg)

The main program may look like -

with open('filename','wb') as f:
    cinstance = CSVLogger(...,f) #file and other parameters
    .... #other logic
    cinstance.write_something("some message")
    ..... #other logic

Though if this complicates thing a-lot, you are better off not using the with statement and rather making sure that you close the file when when its need is over.

Accessing class file from multiple methods in Python

1 Answers1