Lab Exercise 4

Learning Outcomes Covered

    • Using inheritance in OOP

    • Basic file I/O

    • Exception handling

    • System commands

Instructions

    • Unless specified otherwise,

o you are allowed to use anything in the Python Standard Library; o you are NOT allowed to use any third-party library in your code;

o you can assume all user inputs are always valid and your program for each question need not include code to validate them.

    • The underlined blue texts in the Sample Runs section of some questions refer to the user inputs. It does NOT mean that your program needs to underline them.

    • Please name each of your Python script files exactly as the name in parenthesis as shown in each question heading. Marks will be deducted if violating this instruction.


Part I: In-Class Exercise

Question 1: Inheritance (q1.py) (20%)

Write two classes named Person and Programmer so that the following client code runs. Note that the Programmer class must inherit from the Person class, which maintains two attributes, name and age. Programmer has an extra attribute, skillset (a Python list), to describe the skills (e.g. C++, Java, Linux) the programmer acquired. The methods add_skill(s), remove_skill(s) and has_skill(s) add s to, remove s from, and check existence of s in the skillset list, respectively.

Sample Usage:


mike = Person("Mike", 21)

sheila = Programmer("Sheila", 23)

sheila.add_skill("C++")

sheila.add_skill("Python")

sheila.add_skill("Linux")

sheila.remove_skill("Python")

print(mike.name, mike.age)    # Mike 21

print(sheila.name, sheila.age)    # Sheila 23

print(isinstance(sheila, Person)) # True print(isinstance(mike, Programmer)) # False

print(sheila.has_skill("C++"))    # True

print(sheila.has_skill("Python"))    # False

print(sheila.has_skill("Java"))    # False

print(sheila.has_skill("Linux"))    # True



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CSCI1040 Hands-on Introduction to Python, 2020/21 Term 2

Department of Computer Science and Engineering, The Chinese University of Hong Kong


Part II : Take-home Exercise (Deadline: Mar 7, 11:59pm)

Question 2: Inheritance (q2.py) (20%)

Write a small game which consists of two characters, heroes and beasts. Every character has three attributes, namely name, AP (attack points) and HP (health points). A hero attacks a beast and vice versa. The character being attacked will drop in HP by an amount equal to the attacker’s AP.

To implement this game, your program defines the following three classes:


class Character(ABC):

# Your code goes here

class Hero(Character):

# Your code goes here

class Beast(Character):

# Your code goes here

Note that Character is an abstract base class (ABC) with an abstract method attack(), which must be overridden by every subclass. For Character objects a and b, the call a.attack(b) means that a attacks b. The game ends when a character’s HP drops to or below zero.


Hero
Beast
Default HP:
100
70
Default AP:
20
25
Attack behavior:
print "Punching <enemy name> ..."
print "Biting <enemy name> ..."

Your code must pass the sample test as follows:

Sample Client:


try:

c = Character("Dummy", 10, 200) # with HP = 200, AP = 10 except TypeError:

print("Can't instantiate abstract class Character")

hero = Hero('Spider-Man')    # with HP = 100, AP = 20

beast = Beast('Two-Headed')    # with HP = 70, AP = 25

while True:
if hero.HP <= 0:

break

hero.attack(beast)
if beast.HP <= 0:

break
beast.attack(hero)

hero.print_info()

beast.print_info()

print((beast.name if beast.HP <= 0 else hero.name) + " is dead!")


2
CSCI1040 Hands-on Introduction to Python, 2020/21 Term 2

Department of Computer Science and Engineering, The Chinese University of Hong Kong


Sample Output:


Can't instantiate abstract class Character Punching Two-Headed ... Biting Spider-Man ... Punching Two-Headed ... Biting Spider-Man ... Punching Two-Headed ... Biting Spider-Man ... Punching Two-Headed ... Spider-Man: AP = 20, HP = 25 Two-Headed: AP = 25, HP = -10 Two-Headed is dead!






Question 3: File I/O (q3.py) (20%)

Given the file names of two text files, which contains one integer per line. Write a function called merge_and_sort() which merges the contents of the two files, sort all the integers in ascending order, and output the result as a new file. The function’s call signature is given as follows:


def merge_and_sort(input_file_A, input_file_B, output_file):

# Your code goes here

merge_and_sort("a.txt", "b.txt", "c.txt") # Creates c.txt

For example, suppose that file a.txt contains


6

1

3

and file b.txt contains


2

4

5

The above function call will create a new file c.txt which contains:


1

2
3

4
5

6

Normally, all the files are accessible in the current directory (containing the Python script) and they should store one integer per line. However, there is chance that these assumptions are violated. Your code must include the following exception handling when reading an input file:




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CSCI1040 Hands-on Introduction to Python, 2020/21 Term 2

Department of Computer Science and Engineering, The Chinese University of Hong Kong

    • If the input file does not exist or the user has no permission to access it, print the error message "File access error!".

    • If some line(s) in the file is not an integer, print error message “Invalid input!”

In either case, no output file will be generated.

When creating the output file, you may assume that no exception handling is needed (e.g., it is always given a different name from the input files; there is always sufficient disk space for its creation).


Question 4: File I/O (q4.py) (20%)

There is a database log file named attendance.txt, which records students’ course attendance information formatted as follows:


('2020-01-13 11:50:09', 271, 131),

('2020-01-14 10:52:19', 273, 131),

('2020-01-13 11:50:19', 271, 126)

Every line is one record of attendance formatted as (time, course id, student id).

Write a function called reformat_as_dict(), which takes filename as parameter and returns a dictionary with student id as key and a list of reformatted attendance records as value. A reformatted attendance record is a dictionary with two entries, course id and attendance time.


def reformat_as_dict(filename):

# Your code goes here

For example, for the three records above, the function returns the following dictionary:


{131: [{'courseid': 271, 'attendtime': '2020-01-13 11:50:09'}, {'courseid':


273,'attendtime':'2020-01-14 10:52:19'}], 126: [{'courseid': 271, 'attendtime':


'2020-01-13 11:50:19'}]}



The above output is hard to read. So, for the final output of this program, it dumps the dictionary as a JSON file using the following code. Suppose that data_dict is the dictionary returned from the call of reformat_as_dict("attendance.txt").


import json

jstr = json.dumps(data_dict, indent=4, sort_keys=True)

with open("attendance.json", "w") as f:

f.write(jstr)


This code snippet will produce an output file called attendance.json which contains the following neatly indented data (also sorted by key) in JSON format:




4
CSCI1040 Hands-on Introduction to Python, 2020/21 Term 2

Department of Computer Science and Engineering, The Chinese University of Hong Kong


{

"126": [

{

"attendtime": "2020-01-13 11:50:19",

"courseid": 271

}

],

"131": [

{

"attendtime": "2020-01-13 11:50:09",

"courseid": 271,

},

{

"attendtime": "2020-01-14 10:52:19",

"courseid": 273

}

]

}

Note: the provided input file attendance.txt has many more records than the simplified example above. We have provided the expected output attendance.json to ease your program correctness check. You may make use of some text comparison tools like https://www.diffchecker.com or https://text-compare.com to compare your output file content against the provided one.


Question 5: OS Interface (q5.py) (20%)

Write a program which lets the user enter a directory path via console input and finds all the Python script files (i.e., files with file extension ".py") under the specified directory (including all its subfolders, recursively) and creates a text file named python_files.txt to store the relative paths of all the files found.

Hint: import the os module and use os.walk() to walk the directory tree at the specified path. Check the documentation of this method to see how to use it.

We have provided a sample directory whose content is depicted as follows:

q5_dir

├── Sheila
    • ├── Python

    • │   └── hello_world.py
    • ├── hello_world.cpp

    • └── q_learning.py
├── dqn.py

├── ppo.py

└── sum.cpp


Put this directory next to your q5.py script file before running your program.





5
CSCI1040 Hands-on Introduction to Python, 2020/21 Term 2

Department of Computer Science and Engineering, The Chinese University of Hong Kong

Sample Usage (on Linux / macOS):

Example 1:


Input path to search: q5_dir


python_files.txt contains:


q5_dir/dqn.py

q5_dir/ppo.py

q5_dir/Sheila/q_learning.py

q5_dir/Sheila/Python/hello_world.py



Example 2:

Suppose that the sample directory is put under /Users/sheilazheng.


Input path to search: /Users/sheilazheng/q5_dir


python_files.txt contains:


/Users/sheilazheng/q5_dir/dqn.py /Users/sheilazheng/q5_dir/ppo.py /Users/sheilazheng/q5_dir/Sheila/q_learning.py /Users/sheilazheng/q5_dir/Sheila/Python/hello_world.py


Example 3:


Input path to search: q5_dir/Sheila


python_files.txt contains:


q5_dir/Sheila/q_learning.py

q5_dir/Sheila/Python/hello_world.py



Example 4:


Input path to search: a_non-existent_dir


An empty file python_files.txt is generated.











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CSCI1040 Hands-on Introduction to Python, 2020/21 Term 2

Department of Computer Science and Engineering, The Chinese University of Hong Kong




Sample Usage (on Windows):

On a Windows platform, the file separator becomes "\" instead of "/". Below we repeat Example 1 and Example 2 when running the Python program on a Windows platform:

Example 1:


Input path to search: q5_dir


python_files.txt contains:


q5_dir\dqn.py
q5_dir\ppo.py

q5_dir\Sheila\q_learning.py

q5_dir\Sheila\Python\hello_world.py



Example 2:

Suppose that the sample directory is put under C:\Users\sheilazheng.


Input path to search: C:\Users\sheilazheng\q5_dir


python_files.txt contains:


C:\Users\sheilazheng\q5_dir\dqn.py C:\Users\sheilazheng\q5_dir\ppo.py C:\Users\sheilazheng\q5_dir\Sheila\q_learning.py C:\Users\sheilazheng\q5_dir\Sheila\Python\hello_world.py



More Notes:

Hint: os.path.sep may be useful for writing platform-independent code. It returns a right pathname separator on a specific OS platform.

The appearance order of the file paths in your output file need not follow exactly the same as that in our sample output as shown.
















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