How to concatenate files while looping through lists?

[DeaD_EyE]

Maybe this helps a little bit to understand.

import pathlib
from collections import defaultdict
from itertools import groupby

def get_group(file):
    """
    Returns the group as a str
    """
    return '_'.join(file.name.split('_')[2:4])

def file_order(file):
    a, b = map(int, file.name.split('_')[:2])
    # I am using numbers for a and b in test code
    # 
    #a, b = file.name.split('_')[:2]
    return a, b
    
def grouped_files(doc_root):
    doc_root = pathlib.Path(doc_root)
    grouped = defaultdict(list)
    iterator = doc_root.glob('*_*_*_*.hdf')
    for group, items in groupby(iterator, key=get_group):
        for item in items:
            grouped[group].append(item)
    for files in grouped.values():
        # doing an inline sort, which mutates the
        # list
        files.sort(key=file_order)       
    return grouped

If the order of the hdf_files in each group doesn't matter, you can remove the sort code.
On the other side, sorting can simplify the code.

def grouped_files(doc_root):
    doc_root = pathlib.Path(doc_root)
    grouped = {}
    iterator = doc_root.glob('*_*_*_*.hdf')
    sorted_files = sorted(iterator, key=get_group)
    for group, items in groupby(sorted_files, key=get_group):
        # because of the sorted list by group,
        # each group occours only once
        # items is an interator, you have to consume the iterator
        grouped[group] = list(items)
        # if sorting of hdf_files is required
        # grouped[group] = sorted(items, key=file_order)
    return grouped

https://docs.python.org/3/library/stdtypes.html#list.sort
https://docs.python.org/3/library/functions.html#sorted
https://docs.python.org/3/library/iterto...ls.groupby
https://docs.python.org/3/library/pathli...le-pathlib