Fix Python – What is the proper way to determine if an object is a bytes-like object in Python?


Asked By – A. Wilcox

I have code that expects str but will handle the case of being passed bytes in the following way:

if isinstance(data, bytes):
    data = data.decode()

Unfortunately, this does not work in the case of bytearray. Is there a more generic way to test whether an object is either bytes or bytearray, or should I just check for both? Is hasattr('decode') as bad as I feel it would be?

Now we will see solution for issue: What is the proper way to determine if an object is a bytes-like object in Python?


There are a few approaches you could use here.

Duck typing

Since Python is duck typed, you could simply do as follows (which seems to be the way usually suggested):

    data = data.decode()
except (UnicodeDecodeError, AttributeError):

You could use hasattr as you describe, however, and it’d probably be fine. This is, of course, assuming the .decode() method for the given object returns a string, and has no nasty side effects.

I personally recommend either the exception or hasattr method, but whatever you use is up to you.

Use str()

This approach is uncommon, but is possible:

data = str(data, "utf-8")

Other encodings are permissible, just like with the buffer protocol’s .decode(). You can also pass a third parameter to specify error handling.

Single-dispatch generic functions (Python 3.4+)

Python 3.4 and above include a nifty feature called single-dispatch generic functions, via functools.singledispatch. This is a bit more verbose, but it’s also more explicit:

def func(data):
    # This is the generic implementation
    data = data.decode()

def _(data):
    # data will already be a string

You could also make special handlers for bytearray and bytes objects if you so chose.

Beware: single-dispatch functions only work on the first argument! This is an intentional feature, see PEP 433.

This question is answered By – Elizafox

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0