Comment on http://therning.org/magnus/posts/2017-01-31-000-on-mocks-and-stubs-in-python--free-monad-or-interpreter-pattern-.html

no_mocks.py

# I rewrote the code so that you can keep the os calls unchanged and only add yield before it.
# The interpreter is now represented by a class with static methods instead. The class only
# serves as a dictionary of functions.

import collections
import os
import types

Op = collections.namedtuple('Op', ['op', 'args', 'kwargs'])

class Ops:
    def __getattribute__(self, attr):
        return lambda *args, **kwargs: Op(attr, args, kwargs)

def count_chars_of_file(fn):
    os = Ops()
    fd = yield os.open(fn)
    text = yield os.read(fd)
    n = len(text)
    yield os.close(fd)
    return n

class Os:
    @staticmethod
    def open(filename):
        fd = os.open(filename, os.O_RDONLY)
        return fd

    @staticmethod
    def read(fh):
        text = os.read(fh, 10000)
        return text

    @staticmethod
    def close(fh):
        os.close(fh)
        return None

def runProgram(prog):
    ops = Os()
    def runOp(op):
        return ops.__getattribute__(op.op)(*op.args, **op.kwargs)

    try:
        op = prog.send(None)
        while True:
            if isinstance(op, Op):
                r = runOp(op)
                op = prog.send(r)
            elif isinstance(op, types.GeneratorType):
                r = runProgram(op)
                op = prog.send(r)
    except StopIteration as e:
        return e.value

if __name__ == '__main__':
    import sys
    filename = sys.argv[0]
    program = count_chars_of_file(filename)
    output = runProgram(program)
    print(output)

qualified_names.py

# Added namespacing, so the full name of the action is 'os.open'.
# The namespace is passed to the action object, and the interpreter
# is now represented by a dictionary of objects, where the keys are
# the namespaces.

import collections
import os
import types

Op = collections.namedtuple('Op', ['qual', 'op', 'args', 'kwargs'])

class Ops:
    def __init__(self, namespace):
        self.__namespace = namespace

    def __getattribute__(self, attr):
        cls = type(self)
        if attr.startswith('_' + cls.__name__ + '__'):
            return super(cls, self).__getattribute__(attr)
        else:
            return lambda *args, **kwargs: Op(self.__namespace, attr, args, kwargs)

def count_chars_of_file(fn):
    os = Ops('os')
    fd = yield os.open(fn)
    text = yield os.read(fd)
    n = len(text)
    yield os.close(fd)
    return n

class Os:
    @staticmethod
    def open(filename):
        fd = os.open(filename, os.O_RDONLY)
        return fd

    @staticmethod
    def read(fh):
        text = os.read(fh, 10000)
        return text

    @staticmethod
    def close(fh):
        os.close(fh)
        return None

def runProgram(prog):
    ops = dict(os = Os())
    def runOp(op):
        base = ops.get(op.qual, None)
        if not base:
            raise Exception("No handlers for qualifier: " + op.qual)
        opname = op.op
        try:
            f = base.__getattribute__(opname)
        except:
            raise Exception("No handler for {!s}.{!s}".format(op.qual, opname))
        return f(*op.args, **op.kwargs)

    try:
        op = prog.send(None)
        while True:
            if isinstance(op, Op):
                r = runOp(op)
                op = prog.send(r)
            elif isinstance(op, types.GeneratorType):
                r = runProgram(op)
                op = prog.send(r)
            else:
                raise Exception(op)
    except StopIteration as e:
        return e.value

if __name__ == '__main__':
    import sys
    filename = sys.argv[0]
    program = count_chars_of_file(filename)
    output = runProgram(program)
    print(output)