Built-in Functions and Help

Last updated on 2024-09-25 | Edit this page

Estimated time: 25 minutes

Overview

Questions

  • How can I use built-in functions?
  • How can I find out what they do?
  • What kind of errors can occur in programs?

Objectives

  • Explain the purpose of functions.
  • Correctly call built-in Python functions.
  • Correctly nest calls to built-in functions.
  • Use help to display documentation for built-in functions.
  • Correctly describe situations in which SyntaxError and NameError occur.

Use comments to add documentation to programs.

It’s helpful to add comments to our code so that our collaborators (and our future selves) will be able to understand what particular pieces of code are meant to accomplish or how they work

PYTHON 

# This sentence isn't executed by Python.
name = 'Library Carpentry'   # Neither is this comment
# Anything after '#' is ignored.

A function may take zero or more arguments.

We have seen some functions such as print() and len() already but let’s take a closer look at their structure.

An argument is a value passed into a function. Any arguments you want to pass into a function must go into the ().

PYTHON 

print("I am an argument and must go here.")
print()
print("Sometimes you don't need to pass an argument.")

OUTPUT 

I am an argument and must go here.

Sometimes you don't need to pass an argument.

You always need to use parentheses at the end of a function, because this tells Python you are calling a function. Leave the parentheses empty if you don’t want or need to pass any arguments.

Commonly-used built-in functions include max() and min().

  • Use max() to find the largest value of one or more values.
  • Use min() to find the smallest.

Both max() and min() work on character strings as well as numbers, so can be used for numerical and alphabetical comparisons. Note that numerical and alphabetical comparisons follow some specific rules about what is larger or smaller: numbers are smaller than letters and upper case letters are smaller than lower case letters, so the order of operations in Python is 0-9, A-Z, a-z when comparing numbers and letters.

PYTHON 

print(max(1, 2, 3)) # notice that functions are nestable
print(min('a', 'b', max('c', 'd'))) # nest with care since code gets less readable
print(min('a', 'A', '2')) # numbers and letters can be compared if they are the same data type

OUTPUT 

3
a
2

Functions may only work for certain (combinations of) arguments.

max() and min() must be given at least one argument and they must be given things that can meaningfully be compared.

PYTHON 

max(1, 'a')

ERROR 

TypeError                                 Traceback (most recent call last)
Cell In[6], line 1
----> 1 max(1, 'a')

TypeError: '>' not supported between instances of 'str' and 'int'

Function argument default values, and round().

round() will round off a floating-point number. By default, it will round to zero decimal places, which is how it will operate if you don’t pass a second argument.

PYTHON 

round(3.712)

OUTPUT 

4

We can use a second argument (or parameter) to specify the number of decimal places we want though.

PYTHON 

round(3.712, 1)

OUTPUT 

3.7

Use the built-in function help to get help for a function.

Every built-in function has online documentation. You can always access the documentation using the built-in help() function. In the jupyter environment, you can access help by either adding a ? at the end of your function and running it or Hold down Shift, and press Tab when your insertion cursor is in or near the function name.

PYTHON 

help(round)

PYTHON 

round?

OUTPUT 

Help on built-in function round in module builtins:

round(...)
    round(number[, ndigits]) -> number

    Round a number to a given precision in decimal digits (default 0 digits).
    This returns an int when called with one argument, otherwise the
    same type as the number. ndigits may be negative.

Every function returns something.

Every function call produces some result. When the function’s goal is to produce a certain value, we say that the function returns that value. You can think of the function expression as synonymous with that value. You can use a function call in a “sentence” (a line of code) to represent the value it returns.

PYTHON 

lunch_prices = [9.95, 8.00, 12.50]
print(f'The least expensive lunch costs ${min(lunch_prices)}')

OUTPUT 

The least expensive lunch costs $8.00

Even when a function returns nothing, it returns something.

If a function doesn’t have a useful result to return, it usually returns the special value None. Generally speaking, that means the function was meant to carry out a set of instructions, like printing something or modifying variables. You could say these functions are all about the journey, not the destination.

Each line of Python code is executed in order. In this case, the second line call to result returns ‘None’ since the print statement in the previous line didn’t return a value to the result variable.

PYTHON 

result = print('example')
print(f'result of print is {result}')

OUTPUT 

example
result of print is None

Spot the Difference

  1. Predict what each of the print statements in the program below will print.
  2. Does max(len(cataloger), assistant_librarian) run or produce an error message? If it runs, does its result make any sense?

PYTHON 

cataloger = "metadata_curation"
assistant_librarian = "archives"
print(max(cataloger, assistant_librarian))
print(max(len(cataloger), assistant_librarian))

OUTPUT 

metadata_curation
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[2], line 4
      2 assistant_librarian = "archives"
      3 print(max(cataloger, assistant_librarian))
----> 4 print(max(len(cataloger), assistant_librarian))

TypeError: '>' not supported between instances of 'str' and 'int'

Why Not?

Why don’t max and min return None when they are given no arguments?

Both functions require an argument to execute

PYTHON 

print(max())

ERROR 

TypeError: max expected 1 arguments, got 0

Key Points

  • Use comments to add documentation to programs.
  • A function may take zero or more arguments.
  • Commonly-used built-in functions include max, min, and round.
  • Functions may only work for certain (combinations of) arguments.
  • Functions may have default values for some arguments.
  • Use the built-in function help to get help for a function.
  • Every function returns something.