Function fails on empty sequence

Question

I am trying to find the date of the newest file in a directory, based on some Python code I shamelessly found on this site.

def newest_file_in_tree(rootfnewer, extension=".avi"):
    return max(
        (os.path.join(dirname, filename)
        for dirname, dirnames, filenames in os.walk(rootfnewer)
        for filename in filenames
        if filename.endswith(extension)),
        key=lambda fn: os.stat(fn).st_mtime)

This works, but fails on empty directories.

    key=lambda fn: os.stat(fn).st_mtime)
ValueError: max() arg is an empty sequence

I concede that I am a Python beginner, and the last line is way beyond my pay grade.

Can anyone suggest how I can trap the error and prevent the function from crashing?

hiro protagonist · Accepted Answer · 2019-06-06 13:00:48Z

7

max has a default keyword:

m = max([], key=lambda x: abs(x), default='empty...')
print(m) # 'empty...'

the default will be returned if the iterable is empty. without the default this would raise a ValueError.

a different way for you to go would be to use glob in order to list the files:

import os
from glob import glob

def newest_file_in_tree(rootdir, ext='.avi'):
    glob_wildcard = os.path.join(rootdir, '**/*{}'.format(ext))
    glob_gen = glob(glob_wildcard, recursive=True)
    return max(glob_gen, key=lambda fn: os.stat(fn).st_mtime, default='?')

or even more concise with pathlib:

from pathlib import Path

def newest_file_in_tree(rootpath, ext='.avi'):
    return max(Path(rootpath).glob('**/*{}'.format(ext)),
               key=lambda pth: pth.stat().st_mtime, default='?')

edited Jun 6, 2019 at 13:00

answered Jun 18, 2017 at 11:56

hiro protagonist

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2 Comments

cs95 Over a year ago

Cool. Did not know this.

Milliways Over a year ago

Thanks, I tested this and it worked, but I normally use Python 2 (macOS does not come with Python3 by default!) which does not have the default (also not mentioned in Learning Python)

cs95 · Accepted Answer · 2019-06-02 03:07:44Z

4

You can separate out that monstrously pythonic one liner into two separate lines, to make sure that max is called only when you have a non-empty list of files from which you pick the newest one:

def newest_file_in_tree(rootfnewer, extension=".avi"):
    files = [
        os.path.join(dirname, filename)
        for dirname, dirnames, filenames in os.walk(rootfnewer)
        for filename in filenames
        if filename.endswith(extension)
    ]

    if files:
        return max(files, key=lambda fn: os.stat(fn).st_mtime)

    return None

Not as pretty as @hiroprotagonist's solution, but is more readable.

edited Jun 2, 2019 at 3:07

answered Jun 18, 2017 at 11:53

cs95

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3 Comments

cs95 Over a year ago

@TimPietzcker Yesss! Testing the truthiness of an empty list. Updated, thank you.

Tim Pietzcker Over a year ago

I like this because it unclutters the code and is nicely explicit about what is happening.

cs95 Over a year ago

@TimPietzcker Agreed. An even nicer (albeit less pythonic) solution would be removing the nested loops from that comprehension!

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