Find the mean magnitude of values inside unit circle

Some good stuff:

• You're using error-specific except clauses.
• You're using where blocks.

Items you can improve:

• Use a main method. See here for an example; in particular Example 2.

• Don't nest function definitions unnecessarily. Instead of calling c, which calls b which calls a, pass their return values to each other:

  def main(): c(b(a())) 

• Give your functions better names. Give-or-take the cost of verbosity, a function's name should tell you clearly what it does. When in doubt, give everything a clear whole-word name.

• What's the point of b? If sorting the batches is important, leave a comment that explains why.

• Consider learning comprehensions.

• Until you learn classes and dataclasses (or NamedTuples), you can just use tuples to represent sequences of known length. This encodes your assumptions in the structure of the code.

• Not that I like mutating values, but list has an append method; you don't have to say +=[item]. (It's more efficient too.)
  More broadly, it's always a good investment to read the official docs for whatever tools you're using.

• Separate your user-input loop from your file-parsing process. This will probably require you to nest one function definition inside another:

  def a(): while True: try: filename = input('Which data file? ') with open(filename, 'r') as h: return parse_file(h) except IOError: print('No such file or directory was found, the written file can not be opened.'"\n"'Look through misspellings or that the written file is actually in the pathway.') def parse_line(line): batch, x, y, val = line.split(',') # may raise if there's not the right number! return batch, tuple(float(i) # should these all be floats? should batch be a string? for i in (x, y, val)) def parse_file(open_handle): data = {} for line in open_handle: try: batch, triple = parse_line(line) data.setdefault(batch, []).append(triple) except ValueError: print('Some data are not integers in batch',batch,", line", line,"\nThose values are not taken into account\n" ) return data 

• Try to have all your validation in one place:

  def parse_line(line): b_, x_, y_, v_ = line.split(',') batch, x, y, val = strip(b_), float(x_), float(y_), int(v_) if (x**2) + (y**2) <= 1: return batch, (x, y, val) else: raise ValueError("How can you get the correct message to print out?") 

• If you do all your validation early, do you still need the try... except ZeroDivisionError? I have mixed feelings. On the one hand, it looks like it will no longer be able to happen. On the other, I don't want to encourage you to remove a failsafe.

• Right now c has so many layers of indentation, maybe you should break out a separate function for handling a single batch. On the other hand, you can reduce the complexity by using more off-the-shelf stuff:

  def c(data): for batch, samples in data.items(): n = len(samples) if n: total = sum(val for (x, y, val) in samples) average = total / n print(batch, "\t", average)