Author: Ray Luo
Date: 2020-2-6
Know your enemy.
- Sun Tzu
This is a Python code snippet to demonstrate how some possible leap-year bugs could happen in your Python code base.
With such understanding, you are better educated to know what to look for in your code base.
| # Experiencing all the leap year anti-patterns defined in | |
| # https://dev.azure.com/msazure/AzureWiki/_wiki/wikis/AzureWiki.wiki/14194/Leap-Year-Code-Anti-patterns | |
| from datetime import date, datetime | |
| from datetime import timedelta | |
| # Anti-pattern 1: Adding/subtracting/modifying year while keeping the existing month and day | |
| leapday2020 = date(2020, 2, 29) | |
| # leapday2020.replace(year=2019) # It raises ValueError | |
| # Anti-pattern 1a, 6: Assuming all combinatinos of year-month-day components are valid | |
| # nonexist = date(2019, 2, 29) # It raises ValueError | |
| # Anti-pattern 2: Adding or subtracting days when intending to represent a year | |
| first_day_2020 = date(2020, 1, 1) | |
| next_year = first_day_2020 + timedelta(days=365) | |
| print(next_year.year) # It will not be 2021 | |
| # Solution | |
| real_next_year = first_day_2020.replace(year=first_day_2020.year + 1) | |
| print(real_next_year.year) # It will be 2021 | |
| # Anti-pattern 3: Behavior and solution similar to Anti-pattern 2 | |
| # Anti-pattern 4: | |
| days = [0] * 365 | |
| print(len(days)) # Insufficient to store daily data sample in a leap year | |
| # Anti-pattern 5, 7: Detect leap year incorrectly | |
| year = 1900 | |
| is_leap_year = year % 4 == 0 # This is True | |
| if is_leap_year: | |
| print("Is year 1900 really a leap year?") | |
| # date(year, 2, 29) # 1900 is not a leap year | |
| # Anti-pattern 8: Parsing a date string containing only month and day information | |
| print(datetime.strptime("2-28", "%m-%d")) # Year defaults to 1900, so got 1900-2-28 | |
| # print(datetime.strptime("2-29", "%m-%d")) # ValueError |