Lesson 29

Mistakes in ifs

After this lesson: You'll find and fix mistakes in ifs - including the ones that give no message at all.

Video

The video for this lesson is coming soon.

Explanation

Detective Bajtek is back! 🕵️ It's easy to make mistakes in ifs. Some of them Python reports straight away (a red message). But there are also sneaky ones - the program runs, it just gives the wrong answer. Today we'll learn to catch both kinds.

Three clues in an if 🎬

Click Next - three mistakes that Python reports straight away.

Bajtek

Code

n = 5

if n > 0

print("TAK")

🖥️ Screen

SyntaxError: expected ':'

🔍 the : is missing

Clue 1: no colon

Kuba writes if n > 0 - with no colon. Python: SyntaxError: expected ':'. The message says exactly what's missing!

Step 1 / 4

Clue 1 live

The colon is missing - Python runs nothing.

Code

1n = 5
2if n > 0
3    print("TAK")

Screen

File "<lesson>", line 2
    if n > 0
            ^
SyntaxError: expected ':'

1/1 · the program stopped with an error

Clue 2 live

One = in the condition - read Python's hint.

Code

1n = 5
2if n = 5:
3    print("TAK")

Screen

File "<lesson>", line 2
    if n = 5:
       ^^^^^
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

1/1 · the program stopped with an error

Clue 3 live

The line under the if isn't indented.

Code

1n = 5
2if n > 0:
3print("TAK")

Screen

File "<lesson>", line 3
    print("TAK")
    ^^^^^
IndentationError: expected an indented block after 'if' statement on line 2

1/1 · the program stopped with an error

More clues

Code	Message	What to fix
`else` with no colon	`SyntaxError: expected ':'`	add the `:`
`else:` indented differently from `if`	`SyntaxError: invalid syntax`	line else up with the if
`elif` with no if above it	`SyntaxError: invalid syntax`	an elif must come after an if
`If n > 0:`	`SyntaxError: invalid syntax`	if in small letters
`if age > 0:`, but the variable is `aeg`	`NameError: name 'age' is not defined`	the same name everywhere

A mistake with no message 🎬

Click Next - the sneakiest kind of mistake.

Bajtek

Code

if height > 120:

print("TAK")

else:

print("NIE")

🖥️ Screen

TAK

NIE

The program runs... but wrong

Ola writes a program for a ride from 120 cm: if height > 120:. No error! Ola tests 150 - TAK. And 100 - NIE. Does it all work?

Step 1 / 3

Find the bug in the ride 🎢

This program has a sneaky bug. Type a height 📏 - check 120, 119 and 121. Which result is wrong?

Click the arrow - we'll put every comparison on the scale.

Code

1height_str = input()
2height = int(height_str)
3if height > 120:
4    print("TAK")
5else:
6    print("NIE")

Screen

▍

1/2 · line 1 runs next

Sneaky bugs - the detective's list

These bugs give no message - the program runs, but answers wrong:

Suspicious code	What's wrong	The fix
`if height > 120:` for "from 120"	the limit doesn't count	`>=`
`if day == 6 or 7:`	`or 7` is always True	`day == 6 or day == 7`
`if ans == 5:` after `ans = input()`	text is never equal to a number	`int(ans) == 5`
`if p >= 50:` before `elif p >= 90:`	the first True wins too early	the highest limit first
`if n >= 1 or n <= 10:` for "from 1 to 10"	almost any number fits	`and` instead of `or`

The or 7 trap

Day 3 is Wednesday - and the program says Weekend! Look at the switches: 7 is always on.

🔋 ─ ⏻ ─ 💡

Click the arrow - you'll see the condition as a circuit with switches and a bulb.

Code

1day = 3
2if day == 6 or 7:
3    print("Weekend!")
4else:
5    print("School")

Screen

▍

1/4 · line 1 runs next

The fixed or

Now there's a whole question on both sides of or - for Wednesday both switches are off.

🔋 ─ ⏻ ─ 💡

Click the arrow - you'll see the condition as a circuit with switches and a bulb.

Code

1day = 3
2if day == 6 or day == 7:
3    print("Weekend!")
4else:
5    print("School")

Screen

▍

1/4 · line 1 runs next

The detective's trick: print along the way

When you don't know which road the program took, add print("I'm in the if") or print(condition) for a moment - for example print(height > 120). You'll see what Python really checks. Then remove those lines!

Which line has a mistake?

n = 4
if n == 4
    print("four")

What's wrong with this condition: if age = 18:

The program should print TAK for numbers from 1 to 10. Which condition is right?

Which numbers do you type to test the program "from 50 points - passed"?

Remember

Mistakes with a message: no :, = instead of ==, no indentation, a misplaced else.
Read the message: line number + error name + description (Python often gives a hint!).
Sneaky bugs give a wrong answer with no message: a wrong limit, or 7, text instead of a number, the elif order.
Test the limits (119, 120, 121) and every road of the program.
A trick: print(condition) shows what Python really checks.

Now the detective's tasks: each one has a broken program. Find the mistake and submit the fixed version!

Tasks from the video

We solve them together in the video - pause it and try first on your own.

1A lost colon 2A safe with a mistake 3A line with no indentation 4A crooked else 5The ride's limit 6A weekend with a trap 7A riddle with text 8Marks in the wrong order

Practice tasks

Try to solve them without the video. Stuck? Every task has hints and an editorial.

9An else with no colon 10A flipped sign 11or instead of and 12A typo in a name