Finding and choosing tools

What you will be able to do

After this lesson you can

• list the tools of a module with dir,
• read what a tool does with help and its note,
• choose a module for a job, and
• avoid the mistakes that make an import go wrong.

You will not remember every tool

Nobody knows every tool in every module. Experienced programmers look things up all the time. The skill is to find the tool, and to read how it works. Python helps you from inside the program.

Looking inside a module

You can use the same two calls for any module and any tool, even one that you do not know yet. help(len) and help(str.split) work too. When you meet a new tool, read its help before you use it, and then test it on a small case where you know the answer.

The standard library

Python comes with hundreds of modules. This is called the standard library. You have met four. Here are some of the others, so that you know that they exist.

And beyond the standard library, other people share modules that you can install, such as matplotlib for graphs and numpy for fast number work. You will meet them in the next books.

Choosing a tool

When you have a job, ask three questions.

1. Does a tool exist? Almost always yes, for common jobs. Search the name of the job and the word "python".
2. Which module is it in? The table above, and the documentation, tell you.
3. Do I trust it? Read its note, and test it on a small case.

Do not go to the other extreme: a short loop that you understand is better than an import that you do not. But for the square root, the median, or the days between two dates, use the tool. It is correct for all the edge cases that you did not think of, such as leap years.

Mistakes that make an import go wrong

• A wrong module name. ModuleNotFoundError. Check the spelling and the capital letters.
• A tool that does not exist. AttributeError. Check with dir.
• Naming your own file after a module. If your file is called random.py, then import random finds your file, and not the real one. Strange errors follow. Choose names such as dice_game.py.
• Using a tool before the import line.
• **from module import *.** Names clash, and you do not know where a name came from.
• Two imports of the same name. from math import pow and then your own pow replace each other.

Writing this count yourself would take a careful loop. help(math.comb) shows that the tool is already there, with one line of explanation.

Reading a short piece of documentation

Look at this note: math.gcd(*integers) is described as the greatest common divisor of the given integers. It tells you the name, that it takes whole numbers (the star means "any number of them"), and what it gives. The answer is a whole number. With this note, you know that math.gcd(12, 18, 30) is allowed, and gives 6.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab List the tools. You list some tools of a module, and the checker confirms the names.
2. Build. Open the problem Find the tool and the problem Which module?
3. Stretch. Pick a module that you have not used, such as string, run dir and help on two of its tools, and write one sentence about each.

Remember these

• dir(module) lists the names. help(tool) or tool.__doc__ explains one.
• Check a name with in dir(module) before you rely on it.
• Choose a module from the job: numbers, chance, data, dates, files.
• Never name your own file after a module.
• Test a new tool on a small case with a known answer.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How do you list all the tools of math?

2. What does help(math.gcd) do?

3. What goes wrong if you save your program as random.py?