Unit 2: Python fundamentals

Python was created by Guido van Rossum and released in 1991. It is interpreted, high-level, dynamically typed and object-oriented, with readable syntax that uses indentation instead of braces. Application areas: web development (Django, Flask), data science and machine learning (NumPy, pandas, scikit-learn), automation and scripting, desktop apps, game development and IoT.

• Modes: interactive mode (statements run one at a time at the >>> prompt) and script mode (a .py file run with python file.py).

• Keywords: reserved words such as if, else, elif, for, while, def, return, class, import, try, except, lambda, True, False, None (35 in Python 3) — see them with import keyword; keyword.kwlist.
• Identifiers: names for variables, functions and classes; begin with a letter or underscore, may contain digits, are case-sensitive and cannot be keywords.
• Statements: simple (assignment, print) or compound (if, for, def); a long statement can continue on the next line inside brackets or with a backslash.
• Indentation: Python uses indentation (conventionally 4 spaces) instead of braces to mark blocks; inconsistent indentation raises IndentationError.

• Identifiers start with a letter or underscore and are case-sensitive; keywords (if, else, for, while, def, return, class, import, True, False, None …) are reserved.

• Operators: arithmetic (+, −, ×, /, // floor division, % modulus, ** power), comparison, logical (and, or, not), membership (in, not in) and identity (is, is not).
• Type conversion: int("5"), float(3), str(10).
• Input/output: name = input("Enter name: ") (always returns a string) and print("Hello", name, sep=" ", end="\n").

• Precedence (high to low): parentheses, power, unary minus, multiplication and division, addition and subtraction, comparisons, not, and, or.

pythonn = int(input("Number: "))
if n % 2 == 0:
    print("Even")
else:
    print("Odd")

for i in range(1, 6):     # 1 to 5
    if i == 3:
        continue          # skip 3
    print(i)

while True:
    x = int(input())
    if x < 0:
        break             # exit the loop

• pass is a placeholder that does nothing; exit() (sys.exit) ends the program.
• range(start, stop, step) generates numbers up to but not including stop.

python# if-elif-else, nested loop and loop-else
marks = 72
if marks >= 75:   grade = "A"
elif marks >= 60: grade = "B"
else:             grade = "C"

for n in range(2, 20):
    for d in range(2, int(n ** 0.5) + 1):
        if n % d == 0:
            break
    else:                 # runs only if the inner loop did not break
        print(n, end=" ")  # prime numbers

• int (unlimited precision), float (64-bit double), complex (3 + 4j), bool (True, False — a subtype of int). Useful functions: abs(), round(), pow(), divmod(), and modules math, decimal, fractions, random.

pythonmarks = [70, 82, 65]
marks.append(90); marks.insert(0, 55); marks.remove(65); marks.sort()
print(max(marks), min(marks), sum(marks) / len(marks))
student = {"name": "Ana", "roll": 12}
student["course"] = "BCA"
for k, v in student.items():
    print(k, v)

pythona = {1, 2, 3, 4}; b = {3, 4, 5}
print(a | b, a & b, a - b, a ^ b)   # union, intersection, difference, symmetric difference
a.add(9); a.discard(1)
print(3 in a, len(a))
unique = set([2, 2, 3, 3, 4])        # removes duplicates → {2, 3, 4}

• Sets are unordered collections of unique, immutable items; frozenset is an immutable set.

Strings are immutable sequences of characters.

pythons = "Python Programming"
print(s[0], s[-1])        # P g
print(s[0:6])             # Python   (slicing: start included, end excluded)
print(s[::-1])            # reversed string
print(s.upper(), s.lower(), s.split(), s.replace("Python", "C"))
print(len(s), s.find("Pro"), s.count("m"))
name, marks = "Ana", 91.5
print(f"{name} scored {marks:.1f}")      # f-string formatting

• Mutable objects (list, dict, set) can be changed in place; when passed to a function, changes inside the function affect the caller's object.
• Immutable objects (int, float, str, tuple) cannot be changed; "changing" creates a new object.