FreeCampus Python

Choosing a Path with if, elif, and else

Build conditional branches that choose deliberately, cover a fallback, preserve useful results, and distinguish one exclusive decision from several independent checks.
python-foundations decisions-repetition conditionals
Open in Colab
  • Level: Python Foundations · Unit 4
  • Estimated time: 3–5 hours
  • You will learn: Choose among paths with if, elif, and else, order branches safely, and ensure later code has a defined result.
  • Practice in: Google Colab, JupyterLab, or a local editor

Open this lesson in Google Colab

1. Decide whether a ship can depart

A spaceport gate should approve a departure only when its current facts allow it:

fuel = 74
weather = "clear"
permit_active = True

if fuel >= 60 and weather == "clear" and permit_active:
    departure = "approved"
else:
    departure = "hold"

print(departure)

Python evaluates the condition after if. Because it is True, Python runs the first indented block and skips the else block. The name departure refers to "approved" afterward.

A conditional changes which statements execute. It does not make the facts true, repeat the block, or automatically validate every possible input. Your job is to state the rule and provide a useful path for each expected case.

Questions this lesson will answer

  • How do indentation and colons mark a branch?
  • When does an elif condition get evaluated?
  • When should several checks be independent if statements?
  • Why can branch order make a valid rule unreachable?
  • How can later code rely on a result from the decision?

Python tests a branch chain from top to bottom and runs at most one branch.

flowchart TD
  A[Reach the decision] --> B{Fuel below 20?}
  B -- Yes --> C[Set status to critical]
  B -- No --> D{Fuel below 60?}
  D -- Yes --> E[Set status to refuel]
  D -- No --> F[Set status to ready]
  C --> G[Continue after the chain]
  E --> G
  F --> G

2. A colon opens a block and indentation owns its statements

A one-branch conditional runs its body only when the condition is true:

temperature = 2

if temperature <= 3:
    print("Protect the seedlings")

print("Weather check complete")

The first print() belongs to the if because it is indented. The second is aligned with if, so it runs after the decision whether the condition was true or false.

The four visible parts are:

  1. the if keyword;
  2. an expression whose truth is tested;
  3. a colon; and
  4. a consistently indented body.

Omitting the colon is a syntax error. Removing the indentation is also a syntax error because Python expects a block after the header.

WarningIndentation changes behavior

Do not indent a later statement merely because it looks related in English. Its indentation decides whether it belongs to the branch.

is_raining = False

if is_raining:
    print("Take a coat")
    print("This line runs only when it is raining")

print("This line always runs")

3. else supplies the remaining path

else does not have its own condition. It means “none of the earlier branches in this chain ran.”

has_pass = False

if has_pass:
    gate_message = "Welcome aboard"
else:
    gate_message = "Pass required"

print(gate_message)

Exactly one assignment runs, so gate_message exists afterward for both cases. That property matters whenever later code needs a result.

Contrast it with this incomplete decision:

has_pass = False

if has_pass:
    gate_message = "Welcome aboard"

print(gate_message)

The assignment is skipped, then print() tries to use a name that was never bound. Python raises NameError. Either assign a default before the decision or use an else when the two paths are the complete policy.

gate_message = "Pass required"

if has_pass:
    gate_message = "Welcome aboard"

Both designs can be valid. if/else emphasizes two alternatives; a default followed by an override can be clearer when most inputs share the default.

Checkpoint: one or two paths

4. Only the first true branch in a chain runs

Use elif when one decision has more than two mutually exclusive outcomes:

fuel = 42

if fuel < 20:
    fuel_status = "critical"
elif fuel < 60:
    fuel_status = "refuel"
else:
    fuel_status = "ready"

print(fuel_status)

Python asks fuel < 20 first. It is false, so Python evaluates fuel < 60. That is true, so Python assigns "refuel" and skips the rest of the chain. The else means fuel is neither below 20 nor below 60—in other words, it is at least 60.

A useful trace is:

Condition in order Result for fuel = 42 Action
fuel < 20 False continue down chain
fuel < 60 True assign "refuel" and leave chain
else not reached skip

Test exact boundaries, not only typical values:

for fuel in [19, 20, 59, 60]:
    if fuel < 20:
        status = "critical"
    elif fuel < 60:
        status = "refuel"
    else:
        status = "ready"
    print(fuel, status)

20 belongs to "refuel"; 60 belongs to "ready". Changing < to <= would move those endpoints. The operator is part of the policy.

5. Put specific cases before broader cases

Branch order resolves overlaps. Consider this reversed policy:

score = 96

if score >= 70:
    result = "pass"
elif score >= 90:
    result = "distinction"
else:
    result = "retry"

Both comparisons are true for 96, but the first one wins. The distinction branch is unreachable. Put the more specific high threshold first:

if score >= 90:
    result = "distinction"
elif score >= 70:
    result = "pass"
else:
    result = "retry"

This is not a Python preference for large values. It follows only from top-to- bottom, first-true evaluation. For low thresholds, the corresponding safe order may go from smallest to largest, as the fuel example did.

TipRead an elif using what already failed

In elif fuel < 60, the program has already learned that fuel < 20 is false. The effective case is therefore 20 <= fuel < 60, even though the second line does not repeat the lower bound.

6. Independent if statements answer independent questions

An if/elif chain runs at most one branch. Separate if statements may both run:

energy = 18
alerts = []

if energy < 25:
    alerts.append("low energy")

if energy % 2 == 0:
    alerts.append("even calibration value")

print(alerts)

Both facts are true, and both alerts are useful. Replacing the second if with elif would suppress the calibration alert whenever energy is low.

Choose from the relationship between outcomes:

  • one classification: use an if/elif/else chain;
  • several facts can coexist: use independent if statements;
  • one result with a special override: consider a default followed by one or more independent override checks, making precedence explicit.

Checkpoint: branch order and independence

7. Nest only when the second question depends on the first

A nested conditional can reveal dependency:

has_ticket = True
bag_weight = 14

if has_ticket:
    if bag_weight <= 20:
        boarding = "ready"
    else:
        boarding = "check bag"
else:
    boarding = "ticket required"

The weight question matters only after the ticket check succeeds. A compound condition is shorter when only the success case matters:

if has_ticket and bag_weight <= 20:
    boarding = "ready"
else:
    boarding = "not ready"

But the shorter version collapses two useful failure explanations. Choose the shape from the result you need, not from line count.

Avoid a deep staircase of nested branches. When cases form one classification, a flat elif chain is usually easier to scan. When decisions genuinely depend on earlier answers, one or two levels can make the dependency clearer.

8. Common conditions read naturally

Membership can replace repeated equality tests:

command = "launch"

if command in {"launch", "depart", "go"}:
    action = "start engines"
else:
    action = "wait"

Use is None for an intentionally missing value:

assigned_dock = None

if assigned_dock is None:
    message = "Await dock assignment"
else:
    message = f"Proceed to dock {assigned_dock}"

Truthiness is useful only when every falsy value means the same thing:

crew_names = []

if crew_names:
    manifest_status = "crew listed"
else:
    manifest_status = "manifest empty"

Do not use if assigned_dock: if dock 0 is a valid dock. That would confuse a valid falsy integer with missing information. Unit 2’s truthiness rules still apply inside every branch.

9. A conditional expression is for one small value choice

Python can choose between two expressions in one line:

is_night = True
lamp_mode = "on" if is_night else "off"

Read it as: assign "on" when is_night is true, otherwise assign "off". Use this form for one short, readable value choice. Prefer a normal statement when either path has several actions, needs comments, or becomes hard to say aloud. Nested conditional expressions are legal but rarely kind to a beginner or future maintainer.

Checkpoint: expressing the rule clearly

10. Build a spaceport departure gate

Complete the lab from the contract before opening the support. The gate assigns one status and one explanation for every expected input.

fuel = 58
weather = "clear"
permit_active = True
medical_priority = False

status = None
reason = None

Apply these rules in the stated precedence:

  1. A medical-priority flight with an active permit is "priority launch", even when fuel is below the normal threshold, but it still cannot launch in a storm.
  2. A missing permit produces "hold" and reason "permit inactive".
  3. Storm weather produces "hold" and reason "unsafe weather".
  4. Fuel below 60 produces "refuel" and reason "fuel below 60".
  5. Every remaining flight is "approved" with reason "all checks passed".
  6. Add the independent alert "reserve fuel" when fuel is from 50 through 64, inclusive. It can coexist with any status.

After writing the decision, run:

assert status == "refuel"
assert reason == "fuel below 60"
assert fuel_alert == "reserve fuel"

assert status in {"priority launch", "hold", "refuel", "approved"}
assert reason is not None

Then rerun from clean inputs for these cases and record the expected pair before you execute:

  • fuel = 10, clear weather, active permit, medical priority;
  • fuel = 80, storm, active permit, medical priority;
  • fuel = 80, clear weather, inactive permit, no priority; and
  • fuel = 64, clear weather, active permit, no priority.
Hint: separate classification from the independent alert

Use one if/elif/else chain for status and reason. Put the priority case before the ordinary restrictions, but include all facts that priority still requires. Use a separate if/else for fuel_alert because it is not another classification status.

Show one complete solution after attempting the lab 
fuel = 58
weather = "clear"
permit_active = True
medical_priority = False

if medical_priority and permit_active and weather != "storm":
    status = "priority launch"
    reason = "medical priority"
elif not permit_active:
    status = "hold"
    reason = "permit inactive"
elif weather == "storm":
    status = "hold"
    reason = "unsafe weather"
elif fuel < 60:
    status = "refuel"
    reason = "fuel below 60"
else:
    status = "approved"
    reason = "all checks passed"

if 50 <= fuel <= 64:
    fuel_alert = "reserve fuel"
else:
    fuel_alert = "normal fuel band"

The classification chain selects exactly one status. The independent alert then answers a different question and can coexist with that result.

11. Explain the choices

Before leaving the lesson, answer in your own words:

  1. How does alignment show which statements always run?
  2. Why does an else often make a result name safe to use afterward?
  3. What makes a broad condition dangerous before a more specific condition?
  4. When are two independent if statements correct rather than an elif chain?
  5. Why is value is None more precise than not value when zero is valid?

Key points

TipKey points
  • Python evaluates an if/elif chain from top to bottom and runs only the first true branch, or else when no condition succeeds.
  • Colons and indentation define branch ownership; alignment resumes execution after the statement.
  • Put specific overlapping cases before broader ones and test exact boundaries.
  • Use separate if statements when several outcomes may coexist.
  • Ensure every path establishes the result later code needs.
  • Use conditional expressions only for small two-way value choices.

References

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