Built-in Functions and Help
Last updated on 2023-05-02 | Edit this page
- How can I use built-in functions?
- How can I find out what they do?
- What kind of errors can occur in programs?
- 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.
# This sentence isn't executed by Python. = 0.5 # Neither is this - anything after '#' is ignored.adjustment
- We have seen some functions already — now let’s take a closer look.
- An argument is a value passed into a function.
lentakes exactly one.
floatcreate a new value from an existing one.
- Must always use parentheses, even if they’re empty, so that Python knows a function is being called.
print('before') print() print('after')
- Every function call produces some result.
- If the function doesn’t have a useful result to return, it usually
returns the special value
Noneis a Python object that stands in anytime there is no value.
= print('example') result print('result of print is', result)
example result of print is None
maxto find the largest value of one or more values.
minto 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'))
minmust 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.
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'
roundwill round off a floating-point number.
- By default, rounds to zero decimal places.
- We can specify the number of decimal places we want.
- 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.
= 'Hello world!' # creation of a string object my_string 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
- Every built-in function has online documentation.
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.
- 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.
- Won’t even try to run the program if it can’t be parsed.
# Forgot to close the quote marks around the string. = 'Fengname
File "<ipython-input-56-f42768451d55>", line 2 name = 'Feng ^ SyntaxError: EOL while scanning string literal
# An extra '=' in the assignment. = = 52age
File "<ipython-input-57-ccc3df3cf902>", line 2 age = = 52 ^ SyntaxError: invalid syntax
- Look more closely at the error message:
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
- 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.
-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
= 53 age = 100 - aege # mis-spelled 'age'remaining
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.
- 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
= 1.0 radiance = max(2.1, 2.0 + min(radiance, 1.1 * radiance - 0.5))radiance
- Order of operations:
1.1 * radiance = 1.1
1.1 - 0.5 = 0.6
min(radiance, 0.6) = 0.6
2.0 + 0.6 = 2.6
max(2.1, 2.6) = 2.6
- At the end,
radiance = 2.6
- Predict what each of the
max(len(rich), poor)run or produce an error message? If it runs, does its result make any sense?
= "abc" easy_string print(max(easy_string)) = "gold" rich = "tin" poor print(max(rich, poor)) print(max(len(rich), len(poor)))
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'
min return TypeErrors in this case
because the correct number of parameters was not supplied. If it just
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.
name[len(name) - 1]
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.
- Use comments to add documentation to programs.
- A function may take zero or more arguments.
- Commonly-used built-in functions include
- Functions may only work for certain (combinations of) arguments.
- Functions may have default values for some arguments.
- Use the built-in function
helpto 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.