File: day-of-the-week/solution.py

Date: 2026-06-06

Time: 16:09

day-of-the-week/solution.py

Purpose

This file solves LeetCode 1185 — Day of the Week. Given a date as three integers (day, month, year), it returns the English name of that weekday. It's a thin wrapper around Python's datetime.date.weekday().

Key Components

dayofthe_week(day, month, year) -> str — The sole function. Takes integer date components, constructs a datetime.date, and indexes into a lookup table to return the day name.

days list — Maps weekday() return values (0=Monday through 6=Sunday) to their English names. Each string has a trailing space (e.g., "Saturday "), which is a quirk worth noting — this matches the LeetCode problem's expected output format but would be surprising to a caller expecting clean strings.

Patterns

• Standard library delegation: No manual calendar math (Zeller's congruence, etc.). The entire problem reduces to a datetime.date construction + .weekday() call. This is the idiomatic Python approach for date problems — let the stdlib handle leap years and calendar edge cases.
• Lookup table over conditionals: A list indexed by weekday() avoids a chain of if/elif.

Dependencies

Imports: datetime (stdlib). No project-internal dependencies.

Imported by: day-of-the-week/test_solution.py directly. The "Imported By" list in the prompt is misleading — those hundreds of test files import from their own respective solution.py, not from this one.

Flow

1. Caller passes (day, month, year) integers.

2. datetime.date(year, month, day) constructs a date object — note the argument reordering (year first for datetime.date).

3. .weekday() returns an int 0–6 (Monday–Sunday).

4. That int indexes into days, returning the name string.

Single expression, no branching, O(1).

Invariants

• Input range: The docstring states year is 1971–2100, matching the LeetCode constraint. The code itself doesn't validate this — datetime.date accepts a wider range and would raise ValueError on truly invalid dates (month=13, day=32, etc.).
• Trailing whitespace contract: The return value always ends with a space. This is load-bearing for test compatibility.

Error Handling

None. Invalid dates (e.g., Feb 30) propagate as ValueError from datetime.date(). There's no try/except — the function assumes valid input per the LeetCode problem constraints.

Topics to Explore

• [file] day-of-the-week/test_solution.py — See what edge cases are tested (leap years, boundary years 1971/2100)
• [file] day-of-the-year/solution.py — Another date-arithmetic problem; compare whether it also delegates to datetime or uses manual calculation
• [file] number-of-days-between-two-dates/solution.py — A harder date problem that may reveal whether the repo consistently uses datetime or sometimes implements calendar math manually
• [general] trailing-whitespace-convention — Whether other solutions in this repo also return values with trailing spaces, or if this is an anomaly specific to the LeetCode judge's expected output

Beliefs

• day-of-week-uses-stdlib — dayofthe_week delegates all calendar logic to datetime.date.weekday() with no manual computation
• day-of-week-trailing-space — Return values include a trailing space (e.g., "Monday " not "Monday"), matching the LeetCode expected output
• day-of-week-weekday-index-order — The days list is ordered Monday-first (index 0) to match datetime.date.weekday()'s convention, not isoweekday() or strftime
• day-of-week-no-input-validation — The function performs no bounds checking; invalid dates raise ValueError from the datetime.date constructor