Patterns

A pattern is a few lines of code that solve a problem that comes up often.

Look Before You Leap

Look Before You Leap means checking for problems with an if before running a line of code that can raise an exception. The Python glossary calls it LBYL. Each section below names an exception and the checks that prevent it.

IndexError

Check an index against len before using it, and check that a list has items before pop.

scores = [90, 85, 77]
i = 5
if i >= 0 and i < len(scores):
    print(scores[i])
else:
    print(f"No score at index {i}.")

names = []
if len(names) > 0:
    print(names.pop())
else:
    print("No names left.")

No score at index 5.
No names left.

Without the checks, each one crashes on its own:

IndexError: list index out of range
IndexError: pop from empty list

Strings raise the same error, as in Last Character: Empty Input.

ValueError

int needs digits, max and min need at least one item, and remove needs the item to be in the list.

text = "12a"
if text.isdecimal():
    print(int(text) * 2)
else:
    print(f"{text} is not a whole number.")

scores = []
if len(scores) > 0:
    print(max(scores))
else:
    print("No scores yet.")

names = ["Ada", "Grace"]
if "Alan" in names:
    names.remove("Alan")
else:
    print("Alan is not on the list.")

12a is not a whole number.
No scores yet.
Alan is not on the list.

Without the checks, each one crashes on its own:

ValueError: invalid literal for int() with base 10: '12a'
ValueError: max() iterable argument is empty
ValueError: list.remove(x): x not in list

isdecimal is the check that matches int. "²".isdigit() is True, but int("²") crashes (A Bug We Didn't Handle). isdecimal also rejects "-5", which int accepts. Enter a Valid Integer slices off the minus sign before the check.

ZeroDivisionError

Check the number you divide by. An average divides by len(scores), so it needs the list to have items.

total = 60
people = 0
if people > 0:
    print(f"Each pays ${total / people:.2f}")
else:
    print("Nobody to split the bill.")

scores = []
if len(scores) > 0:
    print(f"Average: {sum(scores) / len(scores):.2f}")
else:
    print("No scores yet.")

Nobody to split the bill.
No scores yet.

Without the checks, both divisions crash:

ZeroDivisionError: division by zero

sum([]) is 0 and safe on its own. The average crashes because it divides by len(scores), which is 0.

TypeError

A function that returns None to mean "no answer" needs a check before the caller uses the answer.

def average(scores):
    if len(scores) == 0:
        return None
    return sum(scores) / len(scores)

result = average([])
if result == None:
    print("No scores.")
else:
    print(f"Average: {result:.2f}")

No scores.

Without the check, formatting None crashes:

TypeError: unsupported format string passed to NoneType.__format__

The function checks too: its len check prevents the ZeroDivisionError above. Solution: Average of Scores uses both checks.

FileNotFoundError

os.path.isfile checks that a file exists before open reads it. Appendix K of the textbook covers the os module.

import os

filename = "scores.csv"
if os.path.isfile(filename):
    with open(filename, "r") as f:
        print(f.readline().strip())
else:
    print(f"Could not find {filename}")

When scores.csv is not next to the program:

Could not find scores.csv

Without the check, open crashes:

FileNotFoundError: [Errno 2] No such file or directory: 'scores.csv'

For files, Easier to Ask for Forgiveness than Permission is the safer choice. Another program can delete the file between the check and the open.

Easier to Ask for Forgiveness than Permission

Easier to Ask for Forgiveness than Permission means running a line of code that can raise an exception inside try and handling the exception in except. The Python glossary calls it EAFP. Each section below names an exception and the except that catches it.

IndexError

An index past the end of a list and a pop from an empty list both raise an IndexError.

scores = [90, 85, 77]
i = 5
try:
    print(scores[i])
except IndexError:
    print(f"No score at index {i}.")

names = []
try:
    print(names.pop())
except IndexError:
    print("No names left.")

No score at index 5.
No names left.

A negative index counts from the end of the list, so i = -1 prints 77 with no exception.

ValueError

int raises a ValueError for text that is not a whole number, max and min for an empty list, and remove for an item not in the list.

text = "12a"
try:
    print(int(text) * 2)
except ValueError:
    print(f"{text} is not a whole number.")

scores = []
try:
    print(max(scores))
except ValueError:
    print("No scores yet.")

names = ["Ada", "Grace"]
try:
    names.remove("Alan")
except ValueError:
    print("Alan is not on the list.")

12a is not a whole number.
No scores yet.
Alan is not on the list.

int does the checking itself, so negative numbers like "-5" work.

ZeroDivisionError

Dividing by 0 raises a ZeroDivisionError. An average divides by len(scores), which is 0 for an empty list.

total = 60
people = 0
try:
    print(f"Each pays ${total / people:.2f}")
except ZeroDivisionError:
    print("Nobody to split the bill.")

scores = []
try:
    print(f"Average: {sum(scores) / len(scores):.2f}")
except ZeroDivisionError:
    print("No scores yet.")

Nobody to split the bill.
No scores yet.

A function does not have to catch the exception itself. Python passes it to the line that called the function, and the try around that line catches it:

def average(scores):
    return sum(scores) / len(scores)

try:
    print(f"Average: {average([]):.2f}")
except ZeroDivisionError:
    print("No scores.")

No scores.

FileNotFoundError

open raises a FileNotFoundError when no file has that name.

filename = "scores.csv"
try:
    with open(filename, "r") as f:
        print(f.readline().strip())
except FileNotFoundError:
    print(f"Could not find {filename}")

When scores.csv is not next to the program:

Could not find scores.csv

FileNotFoundError in the exceptions lecture reads every line of the file the same way.