Append an annual total to all Excel files
You are onto a good start, since you already use some functions already; yes they do look like callable objects, but functions in general are referred as such to disambiguate from that more involved construct.

As you noted, you have a number of duplicate code that can be shortened into a single function that take different arguments.  Before doing that, let's take one of the for loops and find out what could be improved first.

The show_results function should take in a parameter, but we can fix this later

Calling os.chdir haphazardly just to list some files is a bit overkill.  I am guessing you want to allow the singular i variable be opened without having to use absolute paths, but doing so can be dangerous if this code is executed in a multi-threaded context (say you want to process multiple files at once), as a thread changing the working directory will change them for all, thus potentially breaking code running on other threads.

Fortunately, Python offers a much better way to glob these files from a path, using a module incidentally named glob.  Now the loop is simply

# insert these into the imports
from os.path import join
from glob import glob

for path in glob(join('path_to_excel_files'. '*.xlsx')):


The reason why we use os.path.join is for cross-platform compatibility.  Also try to avoid single letter names if there is a more expressive name available.

Lastly, the sum call in each loop could be done once after all the values have been gathered rather than every loop as the result would not matter at all.

To simplify code, look for parts that are common, and from that parts that take values that define the specific usage.  From the three for loops we can see that it's just simply sheet identifier within the workbook, the cell identifier from the sheet.  This is what it might look like (please note that I don't have the library installed, nor your Excel files, so I can't exactly test the following)

def process_workbooks_in_dir(excel_root, sheet_id, cell_id):
    values = [] 
    for path in glob(join(excel_root, '*.xlsx')):
        workbook = openpyxl.load_workbook(path, data_only=True)
        sheet = workbook[sheet_id]
        value = sheet[cell_id].value
        values.append(value)

    return values


Note that sum can be called later to sum up the list of values returned, this function now simply process the list of excel files within the provided base path, and the load_workbook function will be given the absolute path of each excel files for the processing to be done.  To use this, the following may be done.

my_list_2015 = process_workbooks_in_dir('path_to_excel_files', 'Blatt1', 'L46') 
my_list_2016 = process_workbooks_in_dir('path_to_excel_files', 'Blatt1', 'L46')
my_list_2017 = process_workbooks_in_dir('path_to_excel_files', 'Zusammen', 'C11')

my_list_total_2015 = sum(my_list_2015) 
my_list_total_2016 = sum(my_list_2016)
my_list_total_2017 = sum(my_list_2017)


Of course, verify that all this still working as intended.  You may note that the 2015 and 2016 lists are using the same parameters - this is now much more visible with the repeated code moved out of the way to show how these parameters can become accidentally (or intentionally) repeated in the remaining code.

While this is basically done, we can still do better with more advanced techniques.



As there is significant overhead with opening and closing files (especially for a complicated file format such as excel spreadsheets), it would be great if this is done only once for all the operations required.  Let's explore for way to streamline the flow better so that this can be done.  Note that the actual act of opening/closing files can be decoupled from the usage of the workbook, and that the workbook can be passed as a parameter.  Let's move that workbook usage logic to a separate function:

def read_value_from_workbook(workbook, sheet_id, cell_id):
    sheet = workbook[sheet_id]
    return sheet[cell_id].value


Now this is a much more simple function, but we still need the other bits such as opening the files, specify how to get the data through this function and to store it.

def process_workbooks_by_spec(excel_root, specification):
    results = {}
    # predefine the results with the year lists as required
    for year, sheet_id, cell_id in specification:
        results[year] = [] 

    # for the actual work
    for path in glob(join(excel_root, '*.xlsx')):
        workbook = openpyxl.load_workbook(path, data_only=True)
        for year, sheet_id, cell_id in specification:
            value = read_value_from_workbook(workbook, sheet_id, cell_id)
            results[year].append(value)

    return results


Note that Python can unpack tuples into multiple variables so we can have a list of tuples put into friendly variable names within the for loop to assign the year, sheet_id and cell_id for usage.  Also another thing that can simplify/eliminate the initial assignment loop is to use collections.defaultdict, which would reduce the code to simply this:

from collections import defaultdict

def process_workbooks_by_spec(excel_root, specification):
    results = defaultdict(list)
    for path in glob(join(excel_root, '*.xlsx')):
        workbook = openpyxl.load_workbook(path, data_only=True)
        for year, sheet_id, cell_id in specification:
            value = read_value_from_workbook(workbook, sheet_id, cell_id)
            results[year].append(value)

    return results


This is now much more efficient in terms of overall execution.  All the required operation needed to extract data can be passed in as a single list that might look something like this:

spec = [
    ('2015', 'Blatt1', 'L46'), 
    ('2016', 'Blatt1', 'L46'),
    ('2017', 'Zusammen', 'C11'),
] 


Note that the top level loop still provide the path to the files, which then open the workbook, then the inner loop unpacks the specification and pass the ids along with the workbook to the inner function, extract the required data and append that to results dictionary under the respective key (by year).

As this is relatively simple, the inner loop remain as is within that function, however for much more complicated business logic, it is wise to consider moving the inner loop outside to a separate function so that the logic is contained in a more concise way.

Now to use it:

results = process_workbooks_by_spec('path_to_excel_files', spec)


Accessing results['2015'] should now return the list of values extracted, and one can simply pass the results into a function that will process this.  Consider modifying show_results to this:

def show_results(results):
    for year, values in sorted(results.items()):
        result = sum(values)
        print("-" * 14 + str(year) + "-" * 14)
        print("Total Sum Netto %s: %.2f" % (year, result)) 
        print("Total Sum Brutto %s: %s" % (year, brutto_total(result)))


Now rather than relying on some global value outside of the function and some hard-coded value defined at the module, this only take in the results dictionary produced by the process_workbooks_by_spec function and iterate through every item and produce an output.  Of course, the sum is done at the time of calculation and if this is undesirable, it can be moved as part of the results as another dictionary, but this is up to you.

I would also not prematurely have some math processing function to return something that is not a number, so brutto_total should return the number as is, and format it later to keep the summary string consistent.  Putting it all together

from os.path import join
from glob import glob
from collections import defaultdict
import openpyxl


def brutto_total(netto):
    return netto + netto * 0.2


def read_value_from_workbook(workbook, sheet_id, cell_id):
    sheet = workbook[sheet_id]
    return sheet[cell_id].value


def process_workbooks_by_spec(excel_root, specification):
    results = defaultdict(list)
    for path in glob(join(excel_root, '*.xlsx')):
        workbook = openpyxl.load_workbook(path, data_only=True)
        for year, sheet_id, cell_id in specification:
            value = read_value_from_workbook(workbook, sheet_id, cell_id)
            results[year].append(value)

    return results


def show_results(results):
    for year, values in sorted(results.items()):
        result = sum(values)
        print("-" * 14 + year + "-" * 14)
        print("Total Sum Netto %s: %.2f" % (year, result))
        print("Total Sum Brutto %s: %.2f" % (year, brutto_total(result)))


spec = [
    ('2015', 'Blatt1', 'L46'),
    ('2016', 'Blatt1', 'L46'),
    ('2017', 'Zusammen', 'C11'),
]

results = process_workbooks_by_spec('path_to_excel_files', spec)
show_results(results)


This is how I might have done this (also reminder: I don't have your files and library installed, but this complete version at least pass the syntax check); hopefully this shows how you might be able to make use of the rich standard library functions that are available in Python to simplify certain tasks, and to also think about how one might break a program down and restructure it into a more clear, reusable form.