How does open context manager work?

[deanhystad]

While answering a question in another thread I wrote this:

import contextlib
 
@contextlib.contextmanager
def CreateFile(FilePath):
    file = open(FilePath, 'w', encoding="utf-8")
    file.write("Court,Location,Citation Number,Case Description,File Date\n")
    yield file
    file.close()

I occasionally create context managers when writing code that requires "cleaning up", but I never did it for a file. To use the code above, I am forced to us a context manager:

with CreateFile(filename) as file:
    # do stuff with file

I cannot do this because CreateFile creates a context object, not a file:

file = CreateFile(filename)

So how does open() let me do this?

file_one = open(filelename_one)
with open(filename_two) as file_two:
    # do file_two stuff

The best I can do to mimic open() is this:

class CreateFile():
    def __init__(self, filename, mode="w", encoding="utf-8"):
        print(f"CreateFile({filename})")
        self.file = open(filename, mode, encoding=encoding)
        self.file.write("Court,Location,Citation Number,Case Description,File Date\n")

    def __enter__(self):
        print("enter")
        return self.file

    def __exit__(self, *_):
        print("exit")
        self.file.close()

    def __getattr__(self, attribute):
        print(f"__getattr__({attribute})")
        return getattr(self.file, attribute)

file = CreateFile("junk.txt")
file.write("This is a test\n")
print(file.tell())
file.close()

print("\n")
with CreateFile("junk2.txt") as file:
    file.write("This is a test\n")

Output:
CreateFile(junk.txt)
__getattr__(write)
__getattr__(tell)
75
__getattr__(close)


CreateFile(junk2.txt)
enter
exit