Python中[] []的含义是什么？ 是pythonic吗？

Question

I'm reading some Python code like this:

import glob
from os.path import *
...
files = glob.glob(prefix + '*') # list out pwd according to some @prefix
res = [f + ['', '/'][isdir(f)] for f in files] # append a `/` to fold

But we can do the append more explicitly:

res = [f + '/' for f in files if isdir(f)]

is the second impl more pythonic ? or which one is more efficient?

---

Anyway, after asking the question, I know the orginal impl:

for every f in files , if if is a directory, then isdir will return True , and is the index of '/' in the first list, or False and f append nothing, and f will append '/' or '' according the return value of isdir

IMHO, I think the original impl makes newbie comfused with the [][] at the first glance.

Answer 1

As noted in the comments, the second implementation is not equivalent. The [,if <filter>] syntax of comprehensions filters out the element from being included at all if the predicate is false.

A more-readable construct might be

[f + '/' if isdir(f) else f for f in files]

Which leverages a conditional expression in lieu of the [a,b][some_boolean] cruft.

Answer 2

The two pieces of code that you present do not perform the same task. The first lists all matching objects, and appends / to those that are directories. The second lists all matching objects that are directories, all with / attached.

So this is something of a non-sequitur. It makes no sense to ask which is more Pythonic or more efficient when they perform different tasks.

Is

['', '/'][isdir(f)]

Pythonic? I'm not sure. In some people's eyes it might be. I personally find it hard to read. At this level of detail much comes down to personal choice. Some people would write the code like that, others would not. You should follow your own instincts.