Built-in Functions and Help
Overview
Teaching: 14 min
Exercises: 10 minQuestions
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.
# 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.
- Functions provide reusable shortcuts to capability that we could probably
implement manually
- Best Practice: Don’t Repeat Yourself
- An argument is a value passed into a function.
len
takes exactly one.int
,str
, andfloat
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
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
andmin
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: unorderable types: str() > 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
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(...)
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.
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 quotation marks around the string.
name = 'Feng
SyntaxError: EOL while scanning string literal
# An extra '=' in the assignment.
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: name 'aege' is not defined
- Fix syntax errors by reading the source and runtime errors by tracing execution.
The Jupyter Notebook has two ways to get help.
- Place the cursor inside the parenthesis of the function,
hold down
shift
, and presstab
. - Or type a function name with a question mark after it.
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
.
result = print('example')
print('result of print is', result)
example
result of print is None
What Happens When
- 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.
- What is the final value of
radiance
?radiance = 1.0 radiance = max(2.1, 2.0 + min(radiance, 1.1 * radiance - 0.5))
Solution
1a.
1.1 * radiance = 1.1
1b.1.1 - 0.5 = 0.6
1c.min(randiance, 0.6) = 0.6
1d.2.0 + 0.6 = 2.6
1e.max(2.1, 2.6) = 2.6
- At the end,
radiance = 2.6
Spot the Difference
- Predict what each of the
- 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
- It throws a TypeError. The command is trying to run
max(4, 'tin')
and you can’t compare a string and an integer
Why Not?
Why don’t
max
andmin
returnNone
when they are given no arguments?Solution
max
andmin
return TypeErrors in this case because the correct number of parameters was not supplied. If it just returnedNone
, 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 stringname
? (Note: we will see a simpler way to do this in a later episode.)Solution
name[len(name) - 1]
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
, andround
.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.The Jupyter Notebook has two ways to get help.
Every function returns something.
Python reports a syntax error when it can’t understand the source of a program.
Python reports a runtime error when something goes wrong while a program is executing.
Fix syntax errors by reading the source code, and runtime errors by tracing the program’s execution.