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Python: The call chain

In this lesson, we'll look at how to combine different approaches when writing code, and we'll also look at common beginner mistakes.

We have the following code:

name = 'Tirion'
print(name.upper().lower())  # => `tirion`

He typed `tirion' on the screen.

This is the first time we have seen the syntax of several consecutive dots, but all the operations we encounter here are familiar. This code combines known features of the language.

This happens a lot in programming. If you don't know the syntax, you can still try combining different approaches, and chances are they will work.

To understand how this code works, you need to break down the chain into separate operations:

name = 'Tirion'
upper_name = name.upper()  # 'TIRION'
print(upper_name.lower())  # 'tirion'

The first and second examples are equivalent. We can perform operations sequentially with intermediate creation of variables, or we can build a continuous chain of attributes and methods. In chains, calculations always go from left to right.

One more example to reinforce:

name = 'Tirion'
print(name.replace('Ti', 'Ki').lower())  # => ?

This code needs a lot of thought. .lower() applies to the result of the method call, which is on the left. And the replace() method returns a string. Newbies often make mistakes in method chains and forget to place a call:

name = 'Tirion'
# This code will not work correctly!

It is also possible to build infinitely long and useless chains that include slices:

# What is the result of such a call?

This will not work with functions, since they are usually nested inside each other - f(f(f()))). This makes the analysis much worse. But that doesn't mean it can't be done nicely. In other languages, this is implemented through a composition of functions or a pipelining operator.


Use the string cutter to get part of the sentence written in the text variable, from 5 to 15 characters inclusive. Process the resulting substring using the .strip() method and display the length of the resulting substring. Perform these operations in a chain without creating intermediate variables.

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.

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