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CMU Libraries Open Data Science Workshops

Built-in Functions and Help

Use comments to add documentation to programs. 

# This sentence isn't executed by Python.
adjustment = 0.5   # Neither is this - anything after '#' is ignored.

A function may take zero or more arguments.

  • We have seen some functions already — now let’s take a closer look.
  • An argument is a value passed into a function.
  • len takes exactly one.
  • int, str, and float create a new value from an existing one.
  • print takes zero or more.
  • print with no arguments prints a blank line.
    • Must always use parentheses, even if they’re empty, so that Python knows a function is being called.
print('before')
print()
print('after')

before

after

Every function returns something.

  • Every function call produces some result.
  • If the function doesn’t have a useful result to return, it usually returns the special value None. None is a Python object that stands in anytime there is no value.
result = print('example')
print('result of print is', result)

example
result of print is None

Commonly-used built-in functions include max, min, and round.

  • Use max to find the largest value of one or more values.
  • Use min to find the smallest.
  • Both work on character strings as well as numbers.
    • “Larger” and “smaller” use (0-9, A-Z, a-z) to compare letters.
print(max(1, 2, 3))
print(min('a', 'A', '0'))

3
0

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

  • max and min must be given at least one argument.
    • “Largest of the empty set” is a meaningless question.
  • And they must be given things that can meaningfully be compared.
print(max(1, 'a'))

TypeError                                 Traceback (most recent call last)
<ipython-input-52-3f049acf3762> in <module>
----> 1 print(max(1, 'a'))

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

Functions may have default values for some arguments.

  • round will round off a floating-point number.
  • By default, rounds to zero decimal places.
round(3.712)

4
  • We can specify the number of decimal places we want.
round(3.712, 1)

3.7

Functions attached to objects are called methods

  • Functions take another form that will be common in the pandas episodes.
  • Methods have parentheses like functions, but come after the variable.
  • Some methods are used for internal Python operations, and are marked with double underlines.
my_string = 'Hello world!'  # creation of a string object

print(len(my_string))       # the len function takes a string as an argument and returns the length of the string

print(my_string.swapcase()) # calling the swapcase method on the my_string object

print(my_string.__len__())  # calling the internal __len__ method on the my_string object, used by len(my_string)


12
hELLO WORLD!
12
  • You might even see them chained together. They operate left to right.
print(my_string.isupper())          # Not all the letters are uppercase
print(my_string.upper())            # This capitalizes all the letters

print(my_string.upper().isupper())  # Now all the letters are uppercase

False
HELLO WORLD
True

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

  • Every built-in function has online documentation.
help(round)

Help on built-in function round in module builtins:

round(number, ndigits=None)
    Round a number to a given precision in decimal digits.

    The return value is an integer if ndigits is omitted or None.  Otherwise
    the return value has the same type as the number.  ndigits may be negative.

The Jupyter Notebook has two ways to get help.

  • Option 1: Place the cursor near where the function is invoked in a cell (i.e., the function name or its parameters),
    • Hold down Shift, and press Tab.
    • Do this several times to expand the information returned.
  • Option 2: Type the function name in a cell with a question mark after it. Then run the cell.

Python reports a syntax error when it can’t understand the source of a program.

  • Won’t even try to run the program if it can’t be parsed.
# Forgot to close the quote marks around the string.
name = 'Feng

  File "<ipython-input-56-f42768451d55>", line 2
    name = 'Feng
                ^
SyntaxError: EOL while scanning string literal

# An extra '=' in the assignment.
age = = 52

  File "<ipython-input-57-ccc3df3cf902>", line 2
    age = = 52
          ^
SyntaxError: invalid syntax
  • Look more closely at the error message:
print("hello world"

  File "<ipython-input-6-d1cc229bf815>", line 1
    print ("hello world"
                        ^
SyntaxError: unexpected EOF while parsing
  • The message indicates a problem on first line of the input (“line 1”).
    • In this case the “ipython-input” section of the file name tells us that we are working with input into IPython, the Python interpreter used by the Jupyter Notebook.
  • The -6- part of the filename indicates that the error occurred in cell 6 of our Notebook.
  • Next is the problematic line of code, indicating the problem with a ^ pointer.

Python reports a runtime error when something goes wrong while a program is executing. 

age = 53
remaining = 100 - aege # mis-spelled 'age'

NameError                                 Traceback (most recent call last)
<ipython-input-59-1214fb6c55fc> in <module>
      1 age = 53
----> 2 remaining = 100 - aege # mis-spelled 'age'

NameError: name 'aege' is not defined
  • Fix syntax errors by reading the source and runtime errors by tracing execution.

What Happens When

  1. Explain in simple terms the order of operations in the following program: when does the addition happen, when does the subtraction happen, when is each function called, etc.
  2. What is the final value of radiance?
radiance = 1.0
radiance = max(2.1, 2.0 + min(radiance, 1.1 * radiance - 0.5))

Solution

  1. Order of operations:
    1. 1.1 * radiance = 1.1
    2. 1.1 - 0.5 = 0.6
    3. min(radiance, 0.6) = 0.6
    4. 2.0 + 0.6 = 2.6
    5. max(2.1, 2.6) = 2.6
  2. At the end, radiance = 2.6

Spot the Difference

  1. Predict what each of the print statements in the program below will print.
  2. Does max(len(rich), poor) run or produce an error message? If it runs, does its result make any sense?
easy_string = "abc"
print(max(easy_string))
rich = "gold"
poor = "tin"
print(max(rich, poor))
print(max(len(rich), len(poor)))

Solution 

print(max(easy_string))

c

print(max(rich, poor))

tin

print(max(len(rich), len(poor)))

4

max(len(rich), poor) throws a TypeError. This turns into max(4, 'tin') and as we discussed earlier a string and integer cannot meaningfully be compared.

TypeError                                 Traceback (most recent call last)
<ipython-input-65-bc82ad05177a> in <module>
----> 1 max(len(rich), poor)

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

Why Not?

Why is it that max and min do not return None when they are called with no arguments?

Solution

max and min return TypeErrors in this case because the correct number of parameters was not supplied. If it just returned None, the error would be much harder to trace as it would likely be stored into a variable and used later in the program, only to likely throw a runtime error.

Last Character of a String

If Python starts counting from zero, and len returns the number of characters in a string, what index expression will get the last character in the string name? (Note: we will see a simpler way to do this in a later episode.)

Solution

name[len(name) - 1]

Explore the Python docs!

The official Python documentation is arguably the most complete source of information about the language. It is available in different languages and contains a lot of useful resources. The Built-in Functions page contains a catalogue of all of these functions, including the ones that we’ve covered in this lesson. Some of these are more advanced and unnecessary at the moment, but others are very simple and useful.