Unit 4: File management and OOP

pythonwith open("notes.txt", "w") as f:      # with closes the file automatically
    f.write("Line 1\n")
with open("notes.txt", "r") as f:
    for line in f:
        print(line.strip())

Pickle saves Python objects to binary files: pickle.dump(obj, f) and obj = pickle.load(f).

pythonimport csv, json
with open("data.csv", newline="") as f:
    for row in csv.reader(f): print(row)
with open("data.json") as f:
    data = json.load(f)                  # json.dump(obj, f) writes

• File object attributes: f.name, f.mode, f.closed, f.encoding. Encoding: open("a.txt", encoding="utf-8") avoids errors with non-English text.

pythonwith open("notes.txt", "w+", encoding="utf-8") as f:
    f.write("Synetic Business School\n")
    print(f.tell())          # current position in bytes
    f.seek(0)                # move to the start
    print(f.read(7))         # Synetic

import os
os.rename("notes.txt", "sbs.txt")
os.remove("sbs.txt")                       # delete a file
os.mkdir("reports")                        # make a directory
print(os.getcwd(), os.listdir("."))        # current directory and contents
os.chdir("reports"); os.chdir("..")
os.rmdir("reports")                        # remove an empty directory

• pathlib offers an object-oriented alternative: Path("reports").mkdir(exist_ok=True).

pythonclass Student:
    college = "SBS"                       # class attribute shared by all objects

    def __init__(self, name, marks):      # constructor
        self.name = name                  # instance attributes
        self.marks = marks

    def grade(self):                      # method
        return "A" if self.marks >= 75 else "B"

    def __str__(self):
        return f"{self.name} ({self.marks})"

s1 = Student("Aman", 82)                  # creating objects
s2 = Student("Riya", 68)
print(s1, s1.grade(), Student.college)

pythons1.marks = 88                             # edit directly
setattr(s2, "marks", 72)                  # edit with a function
print(getattr(s1, "name"), hasattr(s1, "age"))
s1.age = 20                               # add a new attribute to one object
delattr(s1, "age")                        # delete it

• Encapsulation by convention: a single underscore (_fee) marks internal use; a double underscore (__fee) triggers name mangling. Inheritance: class PGStudent(Student) reuses and extends Student.

pythonprint(Student.__name__, Student.__module__, Student.__bases__)
print(s1.__dict__)        # {'name': 'Aman', 'marks': 88}

• Python frees memory automatically. Reference counting: each object counts the references to it; when the count drops to zero, the memory is reclaimed. A cyclic garbage collector (gc module) finds groups of objects that refer only to each other.

pythonimport sys, gc
a = [1, 2, 3]
print(sys.getrefcount(a))   # references (one extra for the call)
b = a; del a                # object still alive through b
del b                       # count reaches zero → freed
gc.collect()                # force a cycle collection

• A destructor __del__(self) runs just before an object is destroyed.