Subclassing file by subclassing `io.TextIOWrapper` — but what signature does its constructor have?

Question

I'm trying to subclass io.TextIOWrapper following this post, although my aims are different. Starting off with this (NB: motivation):

class MyTextIOFile(io.TextIOWrapper):
    def read(self, *args):
        cont = super().read(*args)
        return cont.replace("\x00", "")

I'm trying to open a file using my constructor using

In [81]: f = MyTextIOFile("file.csv")

but this gives:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-90-343e18b2e32f> in <module>()
----> 1 f = MyTextIOFile("file.csv")

AttributeError: 'str' object has no attribute 'readable'

And indeed, it appears io.TextIOWrappers constructor expects to be passed a file object. Through trial and error, I discovered this file object needs to be opened in binary mode. But I can't find documentation anywhere, and I don't feel like building on top of undocumented behaviour (indeed, one attempt to go ahead with it already lead me to problems when trying to pass my object to csv.reader). What is the correct and supported way to subclass a file object in Python 3?

I'm using Python 3.5.0.

Answer 1

I think the documentation you are looking for is

class io.TextIOWrapper(buffer, encoding=None, errors=None, newline=None, line_buffering=False)
    A buffered text stream over a BufferedIOBase binary stream. [...]

The first argument is a binary stream, which implies something opened in binary mode by open.

Answer 2

As far as "fixing" your csv file, you could also use a generator:

# untested
def FixCsv(csv_file, *args, **kwds):
    "assumes text-mode file; removes NUL-bytes"
    if isinstance(csv_file, str):
        file_obj = open(csv_file, *args, **kwds)
    else:
        file_obj = csv_file
    for line in file_obj:
        yield line.replace('\x00','')
    file_obj.close()

But your problem is probably caused by a utf-16 encoded file.