Understanding flow of execution of Python code

I'm trying to do home assignment connected with python from Data Manipulation at Scale: Systems and Algorithms at Curesra. Generally I have problems with understanding base code which was presented as an example of MapReduce alogorythm. I would be grateful for helping me understand it in 2 places, details below.

I tired to go step by step through code flow of below two files after running command:

python wordcount.py 'data/books.json'

1. File wordcount.py is opened
2. mr = MapReduce.MapReduce() - me object is created
3. def __init__(self): part from MapReduce.py is executed
4. We come back to wordcount.py
5. Functions def mapper(record): and def reducer(key,list_of_values): are created but for the time being without execution
6. Python go to if __name__ == '__main__':
7. ` inputdata = open(sys.argv[1]) - json file is assigned to a variable
8. mr.execute(inputdata, mapper, reducer) - A call to the function from MapReduce.py.

And here is my first question we haven't deffined mapper or reducer variable/object so far. Is it just null/no value passed to this function or we somehow defined this variable before but I missed this?

9. Later me move to def execute(self, data, mapper, reducer): in MapReduce.py
10. And there we have mapper(record).

So this is reference to a function in wordcount.py, am I right? But if we have reference to a function in different file shouldn't we use import at the beginning of the file and define from which file this function came?

(...) further code execution

wordcount.py file:

import MapReduce
import sys

"""
Word Count Example in the Simple Python MapReduce Framework
"""

mr = MapReduce.MapReduce()

# =============================
# Do not modify above this line

def mapper(record):
    # key: document identifier
    # value: document contents
    key = record[0]
    value = record[1]
    words = value.split()
    for w in words:
      mr.emit_intermediate(w, 1)

def reducer(key, list_of_values):
    # key: word
    # value: list of occurrence counts
    total = 0
    for v in list_of_values:
      total += v
    mr.emit((key, total))

# Do not modify below this line
# =============================
if __name__ == '__main__':
  inputdata = open(sys.argv[1])
  mr.execute(inputdata, mapper, reducer)

MapReduce.py file:

import json

class MapReduce:
    def __init__(self):
        self.intermediate = {}
        self.result = []

    def emit_intermediate(self, key, value):
        self.intermediate.setdefault(key, [])
        self.intermediate[key].append(value)

    def emit(self, value):
        self.result.append(value) 

    def execute(self, data, mapper, reducer):
        for line in data:
            record = json.loads(line)
            mapper(record)

        for key in self.intermediate:
            reducer(key, self.intermediate[key])

        #jenc = json.JSONEncoder(encoding='latin-1')
        jenc = json.JSONEncoder()
        for item in self.result:
            print jenc.encode(item)

Thank you in advance for help with that.