Python Conditional Statements: if, if‑else, if‑elif‑else, and Nested if

1. Single‑branch statement: if

Syntax:

if <condition>:
    statement block

Notes: the condition is evaluated; if True the block runs, otherwise it is skipped. The block is defined by indentation. A short form can be used, e.g.,

if tag:
    print("True")

. Objects like 0, 0.0, None, empty strings, lists, etc., evaluate to False; everything else evaluates to True. Use the condition directly instead of if tag == True/False.

The pass keyword creates an empty statement that does nothing, useful as a placeholder to keep the program structure valid.

2. Two‑branch statement: if‑else

Syntax:

if <condition>:
    statement block1
else:
    statement block2  # executes when condition is False

Notes: a colon must follow the condition, both blocks must be indented, and else aligns with if. The else clause is optional.

A compact form, the conditional expression, can be written as <expr1> if <condition> else <expr2>, where expr1 is evaluated when the condition is True, otherwise expr2 is evaluated.

3. Multi‑branch statement: if‑elif‑else

Syntax:

if <condition1>:
    block1
elif <condition2>:
    block2
...
elif <conditionN>:
    blockN
else:
    blockN+1

Notes: elif is short for else if and can appear multiple times. Python evaluates conditions sequentially and executes the first block whose condition is True, then exits the whole structure. If none match, the optional else block runs. Place simpler or more frequently true conditions earlier for readability and efficiency.

4. Nested if statements

Nested forms include:

if inside another if

elif containing an if

else containing an if

Notes: there is no syntactic limit on nesting depth, but for maintainability it is advisable not to exceed three levels; consider merging conditions or extracting logic into functions. Pay careful attention to matching indentation to avoid errors.

5. Other tips

Avoid comparing values of different types with == or != because the result may be unintuitive.

When a loop (e.g., for) is nested inside a conditional, ensure proper indentation so the else aligns with the correct block.

Prefer chained comparisons like 0 < a < 10 over combining separate comparisons with logical operators for clarity.