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In http://docs.python.org/library/json.html:

simplejson.load(fp[, encoding[, cls[, object_hook[, parse_float[, parse_int[, parse_constant[, object_pairs_hook[, use_decimal[, **kw]]]]]]]]])

Deserialize fp (a .read()-supporting file-like object containing a JSON document) to a Python object.

I do know what read() and write() do.

But after reading this description "read()-supporting file-like object", I find I don't know what object type supports the read() and write().

And I can't find that in the rest of documentation. Anyone could elaborate more on the statement?

Why I ask this question is for getting "simplejson.load(urllib.open(...))" done.
The return value of "urllib.open(...)" is not a valid object, so I have to tailor it for simplejson. However, it seems like that string is not read()-supporting.

Community
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Kordan Ou
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    Well, you answered your question yourself: really _any_ object that supports proper invocations of either read(), write(), or both, is considered to be a file-like object. It really can be any object you like - the joy of duck typing. – Jim Brissom Dec 05 '10 at 15:43
  • I don't think there's any official standard. Most interfaces should specify exactly the functionality they require. If you want to know if some other thing supports what is needed, you'll have to look at the docs for it or read its source code. – martineau Dec 05 '10 at 18:46
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    "File-like object" is, in fact, [precisely defined](https://docs.python.org/3/glossary.html#term-file-like-object). – chepner Nov 14 '19 at 16:21

5 Answers5

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From the glossary:

file-like object

A synonym for file object

and a file object is

file object

An object exposing a file-oriented API (with methods such as read() or write()) to an underlying resource. Depending on the way it was created, a file object can mediate access to a real on-disk file or to another type of storage or communication device (for example standard input/output, in-memory buffers, sockets, pipes, etc.). File objects are also called file-like objects or streams.

There are actually three categories of file objects: raw binary files, buffered binary files and text files. Their interfaces are defined in the io module. The canonical way to create a file object is by using the open() function.

chepner
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    "with methods such as" ... I suppose that really brings us back to the original question: what _exactly_ is this set of methods? Is e.g. "mode" a required attribute of such an object, or not? – Klaas van Schelven Jan 20 '22 at 12:54
  • @KlaasvanSchelven That, unfortunately, may depend on the underlying operating system or file system. – chepner Jan 20 '22 at 13:05
  • I suppose the question is rhetorical at this point ;-) The point being that we have different opinions on the meaning of "precisely defined" – Klaas van Schelven Jan 20 '22 at 15:05
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In Python, a file object is an object exposing an API having methods for performing operations typically done on files, such as read() or write().

In the question's example: simplejson.load(fp, ...), the object passed as fp is only required to have a read() method, callable in the same way as a read() on a file (i.e. accepting an optional parameter size and returning either a str or a bytes object).

This does not need to be a real file, though, as long as it has a read() method.

A file-like object is just a synonym for file-object. See Python Glossary.

Adrian W
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9

File-like objects are mainly StringIO objects, connected sockets and, well, actual file objects.

If everything goes well, urllib.urlopen() returns a file-like object supporting the necessary methods.

Alistair Carscadden
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ThiefMaster
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7

The IO Class Hierarchy section in the IO documentation contains a table listing the built-in and stub methods for the different types of file-like objects.

Basically, there is a hierarchy of abstract base classes:

To implement a file-like object, you would subclass one of the three descendants of IOBase, but not IOBase itself. See this answer for attempting to determine which of these a given file-like object is.

Each of these classes provides various stub methods and mixins:

Class Stub Methods Mixins
IOBase fileno, seek, truncate close, closed, __enter__, __exit__, flush, isatty, __iter__, __next__, readable, readline, readlines, seekable, tell, writable, writelines
RawIOBase readinto, write read, readall
BufferedIOBase detach, read, read1, write readinto, readinto1
TextIOBase detach, read, readline, write encoding, errors, newlines

The documentation for these methods can be found in the documentation for the classes, linked above.

Phoenix
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1

simplejson has the calls loads and dumps that consumes and produce strings instead of file like objects.

This link has an example in the context of StringIO and simplejson for both file-like and string objects.

http://svn.red-bean.com/bob/simplejson/tags/simplejson-1.3/docs/index.html

kevpie
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    @user313439, you're welcome. Just to be clear, you should be using dumps and loads with strings. Adding StringIO in your situation is extra overhead. – kevpie Dec 06 '10 at 11:42