os.scandir()

Specifically, this PEP proposes adding a single function to the os module in the standard library, scandir, that takes a single, optional string as its argument:

scandir(path='.') -> generator of DirEntry objects

Like listdir, scandir calls the operating system’s directory iteration system calls to get the names of the files in the given path, but it’s different from listdir in two ways:

• Instead of returning bare filename strings, it returns lightweight DirEntry objects that hold the filename string and provide simple methods that allow access to the additional data the operating system may have returned.
• It returns a generator instead of a list, so that scandir acts as a true iterator instead of returning the full list immediately.

scandir() yields a DirEntry object for each file and sub-directory in path. Just like listdir, the '.' and '..' pseudo-directories are skipped, and the entries are yielded in system-dependent order. Each DirEntry object has the following attributes and methods:

• name: the entry’s filename, relative to the scandir path argument (corresponds to the return values of os.listdir)
• path: the entry’s full path name (not necessarily an absolute path) – the equivalent of os.path.join(scandir_path, entry.name)
• inode(): return the inode number of the entry. The result is cached on the DirEntry object, use os.stat(entry.path, follow_symlinks=False).st_ino to fetch up-to-date information. On Unix, no system call is required.
• is_dir(*, follow_symlinks=True): similar to pathlib.Path.is_dir(), but the return value is cached on the DirEntry object; doesn’t require a system call in most cases; don’t follow symbolic links if follow_symlinks is False
• is_file(*, follow_symlinks=True): similar to pathlib.Path.is_file(), but the return value is cached on the DirEntry object; doesn’t require a system call in most cases; don’t follow symbolic links if follow_symlinks is False
• is_symlink(): similar to pathlib.Path.is_symlink(), but the return value is cached on the DirEntry object; doesn’t require a system call in most cases
• stat(*, follow_symlinks=True): like os.stat(), but the return value is cached on the DirEntry object; does not require a system call on Windows (except for symlinks); don’t follow symbolic links (like os.lstat()) if follow_symlinks is False

All methods may perform system calls in some cases and therefore possibly raise OSError – see the “Notes on exception handling” section for more details.

The DirEntry attribute and method names were chosen to be the same as those in the new pathlib module where possible, for consistency. The only difference in functionality is that the DirEntry methods cache their values on the entry object after the first call.

Like the other functions in the os module, scandir() accepts either a bytes or str object for the path parameter, and returns the DirEntry.name and DirEntry.path attributes with the same type as path. However, it is strongly recommended to use the str type, as this ensures cross-platform support for Unicode filenames. (On Windows, bytes filenames have been deprecated since Python 3.3).

os.walk()

As part of this proposal, os.walk() will also be modified to use scandir() rather than listdir() and os.path.isdir(). This will increase the speed of os.walk() very significantly (as mentioned above, by 2-20 times, depending on the system).

Notes on caching

The DirEntry objects are relatively dumb – the name and path attributes are obviously always cached, and the is_X and stat methods cache their values (immediately on Windows via FindNextFile, and on first use on POSIX systems via a stat system call) and never refetch from the system.

For this reason, DirEntry objects are intended to be used and thrown away after iteration, not stored in long-lived data structured and the methods called again and again.

If developers want “refresh” behaviour (for example, for watching a file’s size change), they can simply use pathlib.Path objects, or call the regular os.stat() or os.path.getsize() functions which get fresh data from the operating system every call.

Notes on exception handling

DirEntry.is_X() and DirEntry.stat() are explicitly methods rather than attributes or properties, to make it clear that they may not be cheap operations (although they often are), and they may do a system call. As a result, these methods may raise OSError.

For example, DirEntry.stat() will always make a system call on POSIX-based systems, and the DirEntry.is_X() methods will make a stat() system call on such systems if readdir() does not support d_type or returns a d_type with a value of DT_UNKNOWN, which can occur under certain conditions or on certain file systems.

Often this does not matter – for example, os.walk() as defined in the standard library only catches errors around the listdir() calls.

Also, because the exception-raising behaviour of the DirEntry.is_X methods matches that of pathlib – which only raises OSError in the case of permissions or other fatal errors, but returns False if the path doesn’t exist or is a broken symlink – it’s often not necessary to catch errors around the is_X() calls.

However, when a user requires fine-grained error handling, it may be desirable to catch OSError around all method calls and handle as appropriate.

For example, below is a version of the get_tree_size() example shown above, but with fine-grained error handling added:

def get_tree_size(path):
    """Return total size of files in path and subdirs. If
    is_dir() or stat() fails, print an error message to stderr
    and assume zero size (for example, file has been deleted).
    """
    total = 0
    for entry in os.scandir(path):
        try:
            is_dir = entry.is_dir(follow_symlinks=False)
        except OSError as error:
            print('Error calling is_dir():', error, file=sys.stderr)
            continue
        if is_dir:
            total += get_tree_size(entry.path)
        else:
            try:
                total += entry.stat(follow_symlinks=False).st_size
            except OSError as error:
                print('Error calling stat():', error, file=sys.stderr)
    return total

Naming

The only other real contender for this function’s name was iterdir(). However, iterX() functions in Python (mostly found in Python 2) tend to be simple iterator equivalents of their non-iterator counterparts. For example, dict.iterkeys() is just an iterator version of dict.keys(), but the objects returned are identical. In scandir()’s case, however, the return values are quite different objects (DirEntry objects vs filename strings), so this should probably be reflected by a difference in name – hence scandir().

See some relevant discussion on python-dev.

Wildcard support

FindFirstFile/FindNextFile on Windows support passing a “wildcard” like *.jpg, so at first folks (this PEP’s author included) felt it would be a good idea to include a windows_wildcard keyword argument to the scandir function so users could pass this in.

However, on further thought and discussion it was decided that this would be bad idea, unless it could be made cross-platform (a pattern keyword argument or similar). This seems easy enough at first – just use the OS wildcard support on Windows, and something like fnmatch or re afterwards on POSIX-based systems.

Unfortunately the exact Windows wildcard matching rules aren’t really documented anywhere by Microsoft, and they’re quite quirky (see this blog post), meaning it’s very problematic to emulate using fnmatch or regexes.

So the consensus was that Windows wildcard support was a bad idea. It would be possible to add at a later date if there’s a cross-platform way to achieve it, but not for the initial version.

Read more on the this Nov 2012 python-ideas thread and this June 2014 python-dev thread on PEP 471.

Methods not following symlinks by default

There was much debate on python-dev (see messages in this thread) over whether the DirEntry methods should follow symbolic links or not (when the is_X() methods had no follow_symlinks parameter).

Initially they did not (see previous versions of this PEP and the scandir.py module), but Victor Stinner made a pretty compelling case on python-dev that following symlinks by default is a better idea, because:

• following links is usually what you want (in 92% of cases in the standard library, functions using os.listdir() and os.path.isdir() do follow symlinks)
• that’s the precedent set by the similar functions os.path.isdir() and pathlib.Path.is_dir(), so to do otherwise would be confusing
• with the non-link-following approach, if you wanted to follow links you’d have to say something like if (entry.is_symlink() and os.path.isdir(entry.path)) or entry.is_dir(), which is clumsy

As a case in point that shows the non-symlink-following version is error prone, this PEP’s author had a bug caused by getting this exact test wrong in his initial implementation of scandir.walk() in scandir.py (see Issue #4 here).

In the end there was not total agreement that the methods should follow symlinks, but there was basic consensus among the most involved participants, and this PEP’s author believes that the above case is strong enough to warrant following symlinks by default.

In addition, it’s straightforward to call the relevant methods with follow_symlinks=False if the other behaviour is desired.

DirEntry attributes being properties

In some ways it would be nicer for the DirEntry is_X() and stat() to be properties instead of methods, to indicate they’re very cheap or free. However, this isn’t quite the case, as stat() will require an OS call on POSIX-based systems but not on Windows. Even is_dir() and friends may perform an OS call on POSIX-based systems if the dirent.d_type value is DT_UNKNOWN (on certain file systems).

Also, people would expect the attribute access entry.is_dir to only ever raise AttributeError, not OSError in the case it makes a system call under the covers. Calling code would have to have a try/except around what looks like a simple attribute access, and so it’s much better to make them methods.

See this May 2013 python-dev thread where this PEP author makes this case and there’s agreement from a core developers.

DirEntry fields being “static” attribute-only objects

In this July 2014 python-dev message, Paul Moore suggested a solution that was a “thin wrapper round the OS feature”, where the DirEntry object had only static attributes: name, path, and is_X, with the st_X attributes only present on Windows. The idea was to use this simpler, lower-level function as a building block for higher-level functions.

At first there was general agreement that simplifying in this way was a good thing. However, there were two problems with this approach. First, the assumption is the is_dir and similar attributes are always present on POSIX, which isn’t the case (if d_type is not present or is DT_UNKNOWN). Second, it’s a much harder-to-use API in practice, as even the is_dir attributes aren’t always present on POSIX, and would need to be tested with hasattr() and then os.stat() called if they weren’t present.

See this July 2014 python-dev response from this PEP’s author detailing why this option is a non-ideal solution, and the subsequent reply from Paul Moore voicing agreement.

DirEntry fields being static with an ensure_lstat option

Another seemingly simpler and attractive option was suggested by Alyssa Coghlan in this June 2014 python-dev message: make DirEntry.is_X and DirEntry.lstat_result properties, and populate DirEntry.lstat_result at iteration time, but only if the new argument ensure_lstat=True was specified on the scandir() call.

This does have the advantage over the above in that you can easily get the stat result from scandir() if you need it. However, it has the serious disadvantage that fine-grained error handling is messy, because stat() will be called (and hence potentially raise OSError) during iteration, leading to a rather ugly, hand-made iteration loop:

it = os.scandir(path)
while True:
    try:
        entry = next(it)
    except OSError as error:
        handle_error(path, error)
    except StopIteration:
        break

Or it means that scandir() would have to accept an onerror argument – a function to call when stat() errors occur during iteration. This seems to this PEP’s author neither as direct nor as Pythonic as try/except around a DirEntry.stat() call.

Another drawback is that os.scandir() is written to make code faster. Always calling os.lstat() on POSIX would not bring any speedup. In most cases, you don’t need the full stat_result object – the is_X() methods are enough and this information is already known.

See Ben Hoyt’s July 2014 reply to the discussion summarizing this and detailing why he thinks the original PEP 471 proposal is “the right one” after all.

Return values being (name, stat_result) two-tuples

Initially this PEP’s author proposed this concept as a function called iterdir_stat() which yielded two-tuples of (name, stat_result). This does have the advantage that there are no new types introduced. However, the stat_result is only partially filled on POSIX-based systems (most fields set to None and other quirks), so they’re not really stat_result objects at all, and this would have to be thoroughly documented as different from os.stat().

Also, Python has good support for proper objects with attributes and methods, which makes for a saner and simpler API than two-tuples. It also makes the DirEntry objects more extensible and future-proof as operating systems add functionality and we want to include this in DirEntry.

See also some previous discussion:

• May 2013 python-dev thread where Alyssa Coghlan makes the original case for a DirEntry-style object.
• June 2014 python-dev thread where Alyssa Coghlan makes (another) good case against the two-tuple approach.

Return values being overloaded stat_result objects

Another alternative discussed was making the return values to be overloaded stat_result objects with name and path attributes. However, apart from this being a strange (and strained!) kind of overloading, this has the same problems mentioned above – most of the stat_result information is not fetched by readdir() on POSIX systems, only (part of) the st_mode value.

Return values being pathlib.Path objects

With Antoine Pitrou’s new standard library pathlib module, it at first seems like a great idea for scandir() to return instances of pathlib.Path. However, pathlib.Path’s is_X() and stat() functions are explicitly not cached, whereas scandir has to cache them by design, because it’s (often) returning values from the original directory iteration system call.

And if the pathlib.Path instances returned by scandir cached stat values, but the ordinary pathlib.Path objects explicitly don’t, that would be more than a little confusing.

Guido van Rossum explicitly rejected pathlib.Path caching stat in the context of scandir here, making pathlib.Path objects a bad choice for scandir return values.