Classifying files in a directory by file extension

Question

I have the following code:

resolved = []
rejected = []
for file in files_within("./_input/"):
    file = File(file)
    if file.extension.lower() == "gdb":
        resolved.append(file)
    else:
        rejected.append(file)
MAP["resolved"] = resolved
MAP["rejected"] = rejected

And i would like to improve it to avoid this part:

MAP["resolved"] = resolved
MAP["rejected"] = rejected

But without affecting the performance.

I was thinking about that:

MAP["resolved"] = []
MAP["rejected"] = []
for file in files_within("./_input/"):
    file = File(file)
    if file.extension.lower() == "gdb":
        MAP["resolved"].append(file)
    else:
        MAP["rejected"].append(file)

But i don't know if is a good practice.

Python version 2.6.

Answer 1

Each MAP["resolved"] has to perform a hash on "resolved", and then check up the value in the dictionary's internal structure. This is more costly then looking up a single variable.

Just like in the first, you can assign a local variable, resolved, but you can assign to the dictionary at the same time:

resolved = MAP["resolved"] = []

If you only mutate resolved, then you mutate MAP["resolved"] too. So you can't overwrite resolved with resolved = ['new list'], for example. Which can get you:

MAP["resolved"] = resolved = []
MAP["rejected"] = rejected = []
for file in files_within("./_input/"):
    file = File(file)
    if file.extension.lower() == "gdb":
        resolved.append(file)
    else:
        rejected.append(file)

I am also unsure on what your functions are, I don't remember them being in the standard library, and so I won't suggest any changes to them.

Answer 2

Assuming that:

• you only have two possible results
• you can't have two files with the same name

Then you only need to iterate once and then make the set difference between the results. Something like:

all_files = set([File(file) for file in files_within("./_input/")])
MAP["resolved"] = [file for file in all_files if file.extension.lower() == "gdb"]
MAP["rejected"] = list(all_files - set(MAP["resolved"]))

Note: I didn't test this, it's just the general idea.

EDIT: As pointed out, the original code stores File objects, not file paths.