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Python: Conditional Constructions (if)

In this lesson, we'll look at how you can use conditional constructs to change the behavior of a program, which can be made to act based on the result of a condition it checks. This allows you to write complex programs that behave depending on the situation.

As an example, consider a function that determines the type of sentence passed to it. First, it'll distinguish between normal sentences and questions:

def get_type_of_sentence(sentence):
  last_char = sentence[-1]
  if last_char == '?':
      return 'question'
  return 'normal'

print(get_type_of_sentence('Hodor'))   # => normal
print(get_type_of_sentence('Hodor?'))  # => question

if is a language construct that controls the procedure of how instructions are executed. After the word if it passes a predicate expression, followed by a colon at the end. Next, we pass a block of code. It will execute if the predicate is true.

If the predicate is false, the code block is skipped and the function continues to be executed. In our case, the next line of code, return 'normal', will make the function return a string and terminate.

return can be anywhere in a function, even inside a code block with a condition.


Implement a function called guess_number() that takes a number and checks if the number is equal to a given number (let's say 42). If it's equal, the function should return the string 'You win!', otherwise it should return the string 'Try again!'.

guess_number(42) # You win!
guess_number(61) # Try again!
The exercise doesn't pass checking. What to do? 😶

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

  • Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

I've read the lessons but nothing is clear 🙄

It's hard to make educational materials that will suit everyone. We do our best but there is always something to improve. If you see a material that is not clear to you, describe the problem in “Discussions”. It will be great if you'll write unclear points in the question form. Usually, we need a few days for corrections.

By the way, you can participate in courses improvement. There is a link below to the lessons course code which you can edit right in your browser.


  • Conditional Design. the way to set a condition for code execution. For example, if x > 10: .

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