How can I use isinstance to determine the 'type' of a file object, such as in the expression:
>>> open(file)
5 Answers
These are for use with isinstance.
In Python 3.x, normal file objects are of type io.TextIOWrapper:
>>> type(open('file.txt'))
<class '_io.TextIOWrapper'>
>>> from io import TextIOWrapper
>>> isinstance(open('file.txt'), TextIOWrapper)
True
In Python 2.x, all file objects are of type file:
>>> type(open('file.txt'))
<type 'file'>
>>> isinstance(open('file.txt'), file)
True
4 Comments
user2357112
Most of the time, you don't really care, though. You care that it's an iterable of lines, or that it has a
read method or a write method, or something else relating to the file interface. That's what you should be checking, usually by simply trying to use the object like a file and catching the error if it doesn't support the interface you need.
Rick
FYI, open a file in binary mode
open('file.txt', 'rb') results in <class '_io.BufferedReader'> type.
AndrewWhalan
TextIOWrapper is what is returned if you open in default/ text mode, BufferedReader is what you get in binary ('b') mode, so you can check for either individually or just use their superclass IOBase if you want to catch bothHere is how to type hint a file object.
For type hints only
For type hints only, I would consider typing.TextIO for text files, typing.BinaryIO for binary files, and typing.IO for nonspecific files.
Note that these will not work with isinstance!
For isinstance
For use with isinstance, one can pick their preference from the IO class hierarchy. This answer builds upon the previous answer by user2555451 and its comments. The one-line summary is at the end of the answer.
For text files, io.TextIOBase can be acceptable:
>>> import io
>>> type(f := open('/tmp/foo', 'w'))
<class '_io.TextIOWrapper'>
>>> isinstance(f, io.TextIOBase)
True
>>> f.__class__.__bases__
(<class '_io._TextIOBase'>,)
>>> f.__class__.__mro__
(<class '_io.TextIOWrapper'>, <class '_io._TextIOBase'>, <class '_io._IOBase'>, <class 'object'>)
For text files, avoid io.TextIOWrapper because it does not also work for io.StringIO:
>>> isinstance(io.StringIO('foo'), io.TextIOWrapper)
False
>>> isinstance(io.StringIO('foo'), io.TextIOBase)
True
For binary files, io.BufferedIOBase can be acceptable:
>>> import io
>>> type(f := open('/tmp/foo', 'wb'))
<class '_io.BufferedWriter'>
>>> isinstance(f, io.BufferedIOBase)
True
>>> f.__class__.__bases__
(<class '_io._BufferedIOBase'>,)
>>> f.__class__.__mro__
(<class '_io.BufferedWriter'>, <class '_io._BufferedIOBase'>, <class '_io._IOBase'>, <class 'object'>)
For binary files, avoid io.BufferedReader or io.BufferedWriter because they do not also work for io.BytesIO:
>>> isinstance(io.BytesIO(b'foo'), io.BufferedReader)
False
>>> isinstance(io.BytesIO(b'foo'), io.BufferedWriter)
False
>>> isinstance(io.BytesIO(b'foo'), io.BufferedIOBase)
True
To support both text and binary files, io.IOBase is acceptable:
>>> import io
>>> isinstance(open('/tmp/foo', 'w'), io.IOBase)
True
>>> isinstance(open('/tmp/foo', 'wb'), io.IOBase)
True
In summary, for use with isinstance, I would typically pick io.TextIOBase for text files, io.BufferedIOBase for binary files, and io.IOBase for nonspecific files.
Comments
As stated in the documentation for open:
Open a file, returning an object of the
filetype described in section File Objects.
Thus, open returns a file, and you should use isinstance(foo, file)
Comments
Its type is file. You can tell by the output of type(open("file","w"))
1 Comment
Carsten
This is outdated.
Beware of 'file-like' objects
Unless you know exactly that the code sending you that file-ish object will use standard manners of producing them, any assumption about the type will be risky.
That is because Python frequently uses the notion of 'file-like' objects: Objects that may indeed be something else entirely, but support enough of the file operations for a given purpose -- possibly nothing but read().
They are mentioned for instance at
module io,
shutil.copyfileobj,
zipfile.ZipFile,
and even in the
Python glossary.
For a possible solution, see the discussion of typing.Protocol at https://stackoverflow.com/a/78880231 and
https://stackoverflow.com/a/76575571.
typing(e.g.from typing import TextIOin the "preamble" and thenfile_handle : TextIO = open(...)in your code.