Lesson 3 — Language Rules

Python Syntax

Python's syntax is minimal but precise. The indentation rule, colons, and f-strings are the three things you must internalise completely.

Indentation is Syntax

Every code block in Python — if statements, loops, functions, classes — is defined by indentation, not braces. The standard is 4 spaces per level. Mixing tabs and spaces causes an IndentationError; inconsistent indentation causes silent logic errors where the wrong code runs inside the wrong block.

PYTHON — Indentation 
mark = 74

# Correct indentation:
if mark >= 50:
    print("You passed.")    # 4 spaces
    print("Well done.")     # same level — both inside the if
else:
    print("Not yet competent.")  # back to same level as if

print("Outside the if.")   # zero indentation — always runs

# WRONG — this is a logic error (not a syntax error):
# if mark >= 50:
#     print("You passed.")
#   print("This runs regardless!") # IndentationError or wrong logic

The Colon

Every control structure header ends with a colon: if, elif, else, for, while, def, class. Forgetting the colon is one of the most common SyntaxErrors.

PYTHON — Colons everywhere 
# All of these require a colon at the end of the header:
if True:
    pass

for i in range(5):
    pass

while False:
    pass

def my_function():
    pass

class MyClass:
    pass

f-strings

f-strings (formatted string literals) are the modern, preferred way to embed variables and expressions in strings. They are more readable than concatenation and faster than .format().

PYTHON — f-strings 
name   = "Thandi"
age    = 21
mark   = 74.5
course = "Python"

# f-string — prefix with f, embed expressions in {}:
print(f"Hello, {name}. You are {age} years old.")
print(f"Studying {course.upper()} with a mark of {mark:.1f}%")
print(f"In 5 years you will be {age + 5}.")   # expressions work too

# Alignment and formatting:
for i, n in enumerate(["Java","C#","Python","SQL"], 1):
    print(f"{i}. {n:<10} — available at Your IT Tutor")

# Multi-line f-string:
report = (
    f"Learner: {name}\n"
    f"Course:  {course}\n"
    f"Mark:    {mark:.1f}%\n"
)
print(report)

Comments

PYTHON — Comments and docstrings 
# Single-line comment

"""
Multi-line string used as a comment.
When at the start of a function or class, it is a docstring.
"""

def greet(name):
    """Return a greeting for the given name."""
    return f"Sawubona, {name}!"

Practice Task

Your Turn

Write a script with your name, age, city as variables. Use if/elif/else (with colons and correct indentation) to print different messages for age under 20, 20-25, and over 25. Use f-strings for all output. Add a comment explaining each section.

Common Mistakes

  • Mixing tabs and spaces — use 4 spaces everywhere, never tabs.
  • Forgetting the colon after if, for, def, class.
  • Wrong indentation level — code runs in the wrong block silently.
  • == for comparison, = for assignment — same as Java/C#.

Professional Tip

Set your editor to display whitespace characters so you can see exactly what is a space and what is a tab. VS Code does this with View → Render Whitespace.

Mini Quiz

What is the modern preferred way to embed variables in Python strings?