issue_comments: 1204300671

I talked with @wroberts4 about this in person and if we're not missing some reason to not .seek(0) on a data source then this seems like a simple convenience and user experience improvement. We were thinking maybe it would make more sense to change the function to look like:

python def read_magic_number_from_file(filename_or_obj, count=8) -> bytes: # check byte header to determine file type if isinstance(filename_or_obj, bytes): magic_number = filename_or_obj[:count] elif isinstance(filename_or_obj, io.IOBase): if filename_or_obj.tell() != 0: filename_or_obj.seek(0) # warn about re-seeking? magic_number = filename_or_obj.read(count) filename_or_obj.seek(0) else: raise TypeError(f"cannot read the magic number form {type(filename_or_obj)}") return magic_number

Additionally, the isinstance check is for io.IOBase but that base class isn't guaranteed to have a .read method. The check should probably be for RawIOBase:

https://docs.python.org/3/library/io.html#class-hierarchy

@kmuehlbauer @lamorton I saw you commented on the almost related #3991, do you have any thoughts on this? Should we put a PR together to continue the discussion? Maybe the fsspec folks (@martindurant?) have an opinion on this?