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Concepts·Certified Entry-Level Python Programmer

Python if, elif and else

Objective 2.1 is one line of blueprint - make decisions and branch the flow with the if instruction - and part of the densest 29% in PCEP. The syntax is small: `if`, `if`/`else`, `if`/`elif`/`else`, nesting, and the conditional expression. What gets asked is subtler: which branch runs when two conditions are both true, what counts as a true condition, and what an `if` with nothing in its body does.

Control Flow Guide 10 of 26 Beginner

Written against the versions above. Unchanged since Python 3.0. Python 3.10 added a `match` statement, which is a structural pattern matcher rather than a switch and is **not** on either syllabus - if a question offers `switch` or `case` as Python keywords, neither is one. See {{guide:keywords-indentation-and-comments}} on soft keywords.

One machine, and any shell with Python 3 will do - these exams test the language, not a distribution.
Server NameIP AddressOSRolesCPURAMHDD
RUNNER01192.168.0.27Ubuntu 26.04 LTSPython 3.14.4 - the only machine this path needs2 Core4 GB50 GB

Before you start

  1. The three forms

    There are exactly three, and the third is the second with extra branches in the middle.

    if alone runs its block or does not. if/else chooses one of two. if/elif/else chooses one of many, and the final else is optional even here.

    Every branch header ends in a colon and owns an indented block.

    bash Example session 
    cat > ~/py/ifforms.py <<'PY'n = 7 if n > 5:    print("just if") if n > 100:    print("not this")else:    print("if-else took the else") if n < 0:    print("negative")elif n == 0:    print("zero")elif n < 10:    print("single digit")else:    print("ten or more")PYpython3 ~/py/ifforms.pyjust ifif-else took the elsesingle digit

    Expected resultjust if, if-else took the else, single digit.

    Success conditionYou can write all three forms and trace which block runs.

  2. elif order decides the answer

    This is the trap in objective 2.1, and it is asked far more often than the syntax. Look at classify below and work out what classify(500) returns.

    500 is greater than 100. It is also greater than 0.

    bash Example session 
    cat > ~/py/elifordering.py <<'PY'def classify(n):    if n > 0:        return "positive"    elif n > 100:        return "over a hundred"    else:        return "zero or negative"  print(classify(500))print(classify(5))print(classify(-5))PYpython3 ~/py/elifordering.pypositivepositivezero or negative

    Expected resultpositive - not over a hundred.

    Success conditionYou can spot an unreachable elif branch by reading the order.

  3. A condition does not have to be a comparison

    if evaluates whatever you give it for truthiness - the same rule as bool() from guide 8. There is no requirement that it be a comparison, and no requirement that it be a boolean at all.

    Seven values, each used directly as a condition.

    bash Example session 
    cat > ~/py/iftruthy.py <<'PY'for value in (0, 1, "", "0", [], [0], None):    if value:        print(repr(value), "-> took the if")    else:        print(repr(value), "-> took the else")PYpython3 ~/py/iftruthy.py0 -> took the else1 -> took the if'' -> took the else'0' -> took the if[] -> took the else[0] -> took the ifNone -> took the else

    Expected result0, '', [] and None take the else; 1, '0' and [0] take the if.

    Success conditionYou can use any value as a condition and predict the branch.

  4. Nesting, and what determines it

    An if can contain another if. Nothing marks the nesting except indentation - there is no end and no closing brace - so which else belongs to which if is decided entirely by how far the line is indented.

    bash Example session 
    cat > ~/py/nested.py <<'PY'n = 7 if n > 5:    if n > 10:        print("over ten")    else:        print("between six and ten")else:    print("five or less")PYpython3 ~/py/nested.pybetween six and ten

    Expected resultbetween six and ten.

    Success conditionYou can read a nested branch and match each else to its if.

  5. The conditional expression

    Python's one-line branch is A if condition else B. It is an expression - it produces a value - which is why it can sit on the right of an assignment where a statement cannot.

    Note the order: the *result* comes first, then the condition. Most languages put the condition first.

    bash Example session 
    cat > ~/py/ternary.py <<'PY'x = 5label = "big" if x > 3 else "small"print(label)print("even" if x % 2 == 0 else "odd")print(("small", "big")[x > 3])PYpython3 ~/py/ternary.pybigoddbig

    Expected resultbig, odd, big.

    Success conditionYou can read and write a conditional expression.

  6. An if body cannot be empty

    A colon promises an indented block, and a comment is not a statement - the lexer removes comments before the parser sees them, so a block containing only a comment is an empty block.

    The fix is pass.

    bash Example session 
    printf 'if True:\n# nothing here\nprint("after")\n' > ~/py/emptyif.py && python3 ~/py/emptyif.py  File "/home/sysadmin/py/emptyif.py", line 3    print("after")    ^^^^^IndentationError: expected an indented block after 'if' statement on line 1[exit 1]cat > ~/py/passif.py <<'PY'if True:    passprint("pass filled the block")PYpython3 ~/py/passif.pypass filled the block

    Expected resultIndentationError: expected an indented block after 'if' statement on line 1, then the pass version running cleanly.

    Success conditionYou can satisfy an if that has nothing to do yet.

  7. What the exam does with this objective

    Objective 2.1 is roughly half of block 2's 29%, so around four or five questions. The shapes:

    "Which branch runs?" - given a chain where the value matches two conditions. Top to bottom; the first true one.

    "What is wrong with this code?" - else if instead of elif, a missing colon, or an empty block.

    "Which else belongs to which if?" - read the indentation columns.

    "Is this condition true?" - a bare value rather than a comparison; apply the truthiness rule.

    "Rewrite this as one line." - the conditional expression, with the result before the condition.

    The single most valuable habit is to read the order of an elif chain before reading its logic, because an unreachable branch is invisible if you read the conditions individually.

    guide 11 is next, for the other half of block 2.

    bash 
    rm -f ~/py/ifforms.py ~/py/elifordering.py ~/py/iftruthy.py ~/py/nested.py ~/py/ternary.py ~/py/emptyif.py ~/py/passif.py && ls -A ~/py

    Expected resultAn empty scratch directory.

    Success conditionYou can answer every branching question shape this objective produces.

Troubleshooting

Official sources

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