ይህ ትምህርት ገና ወደ አማርኛ አልተተረጎመም፤ ስለዚህ በእንግሊዝኛ ቀርቧል። የእንግሊዝኛውን ገጽ ክፈቱ

if, elif, else

4 min read

In the last lesson you asked Python yes-or-no questions. Now you'll act on the answers: run some lines only when a condition is true, choose between two paths, or pick one of many. By the end you'll know exactly which path Python takes, including the ordering bug that breaks most grade calculators.

Running lines only sometimes

What does this print? The fare is 180.

ውጤቱን ገምቱ

Decide before you look. Guessing wrong is how this sticks.

fare = 180
if fare > 200:
    print("Too expensive")
print("Done")

Python prints

Done

fare > 200 is False, so Python skips the indented line under the if. print("Done") isn't indented, so it isn't part of the if and runs either way.

An if asks a question. When the answer is True, Python runs the indented lines under it; when it's False, Python skips them. The line with if ends with a colon, and the lines that belong to it are indented by four spaces. Those lines are a blockblockLines indented under a line that ends with a colon. They run together, as one group.. The block ends where the indentation ends.

Indentation isn't decoration in Python; it's how Python knows what belongs to the if.

የተለመደ ስህተት

This breaks

fare = 250
if fare > 200:
print("Too expensive")

Python says

IndentationError: expected an indented block after 'if' statement on line 2

The fix

fare = 250
if fare > 200:
    print("Too expensive")

After a colon, Python expects at least one indented line. It even names the if it was waiting on. Forgetting the colon itself gives SyntaxError: expected ':'.

Two paths, then many

else gives the other path: its block runs when the if condition is False. For more than two paths, add elif ("else if") branches between them. Python checks the conditions from the top and runs only the first block whose condition is true. Step through it and watch which lines Python visits.

One path through the chain

Press Next to run one line at a time and watch each name change.Each row is one step: the line that just ran, and every name's value after it.

1score = 72
2if score >= 85:
3    print("A")
4elif score >= 70:
5    print("B")
6elif score >= 50:
7    print("C")
8else:
9    print("F")

Names and their values after line 1
Name	Value after line 1
score	72new

Output so far

Step 1 of 4

Every step: the line that ran and each name's value after it
Step	Line	score	Printed
1	1	72
2	2	72
3	4	72
4	5	72	B

72 is also at least 50, but line 6 is never checked. Once a branch runs, Python jumps past the whole chain. That makes the order of the conditions matter. This grade scale is an example (universities differ); predict what a score of 90 gets.

ውጤቱን ገምቱ

Decide before you look. Guessing wrong is how this sticks.

score = 90
if score >= 50:
    print("C")
elif score >= 70:
    print("B")
elif score >= 85:
    print("A")

Python prints

90 >= 50 is True, and it's checked first, so Python prints C and skips the rest of the chain. The A branch is never reached.

Every score of 50 or more stops at the first branch. The fix is to ask the hardest question first: order the checks from the highest grade down.

score = 90
if score >= 85:
    print("A")
elif score >= 70:
    print("B")
elif score >= 50:
    print("C")
else:
    print("F")

Ctrl or Cmd + Enter runs

Try 85, 84, 50, and 49. A boundary is where bugs hide.

ጥያቄ

temp is 30. The code is: if temp > 20: print Warm, elif temp > 25: print Hot, else: print Cold. What prints?

An if is checked once

An if isn't a rule that keeps watching a value. It's checked once, when Python reaches it. Step through this:

Step through it

Press Next to run one line at a time and watch each name change.Each row is one step: the line that just ran, and every name's value after it.

1balance = 100
2if balance < 50:
3    print("Low balance")
4balance = 20
5print("Balance:", balance)

Names and their values after line 1
Name	Value after line 1
balance	100new

Output so far

Balance: 20

Step 1 of 4

Every step: the line that ran and each name's value after it
Step	Line	balance	Printed
1	1	100
2	2	100
3	4	20
4	5	20	Balance: 20

When line 2 ran, balance was 100, so the warning was skipped. Line 4 drops it to 20, but Python never goes back up. To check again, you'd need another if after line 4.

An if inside an if

The Ethiopian calendar has 12 months of 30 days, then Pagume, the 13th month, with 5 days, or 6 in a leap year. An Ethiopian year is a leap year when dividing it by 4 leaves 3, so year % 4 == 3 (lesson 1.2's %). 2019 is one. A block can hold another if:

month = 13
year = 2019
if month <= 12:
    days = 30
else:
    if year % 4 == 3:
        days = 6
    else:
        days = 5
print(days)

Ctrl or Cmd + Enter runs

It works, but each level pushes the code further right. When an else holds nothing but another if, join them into an elif:

month = 13
year = 2019
if month <= 12:
    days = 30
elif year % 4 == 3:
    days = 6
else:
    days = 5
print(days)

Ctrl or Cmd + Enter runs

Same answers, one level flatter. Change the year to 2020, then the month to 5.

መልመጃ

Choose a data bundle

Ask how many MB someone needs and print the smallest bundle that covers it. The bundles are examples: Small is 100 MB for 10 birr, Medium is 500 MB for 35 birr, Large is 2,000 MB for 100 birr. Above 2,000 MB, print that no bundle is big enough. Test 100, 101, 2000, and 2500 too.

Output

How many MB do you need? 700
Large: 2,000 MB for 100 birr

Your code

mb = int(input("How many MB do you need? "))
# Check from the smallest bundle up

Ctrl or Cmd + Enter runs

Input

One answer per line, given to input() in order.

መፍትሄውን አሳይ

This is one way to solve it, not the only one. If yours prints the same thing, it works.

mb = int(input("How many MB do you need? "))
if mb <= 100:
    print("Small: 100 MB for 10 birr")
elif mb <= 500:
    print("Medium: 500 MB for 35 birr")
elif mb <= 2000:
    print("Large: 2,000 MB for 100 birr")
else:
    print("No bundle is big enough")

ዋና ዋና ነጥቦች

if runs its indented block only when its condition is True; the colon and the indentation are required.
In an if / elif / else chain, only the first true branch runs, so order matters.
Order grade checks from the highest down, and test every boundary.
An if is checked once, when Python reaches it; changing a value later doesn't rerun it.
When an else holds only another if, flatten it into elif.