Relative Dates Library

This Python library provides intuitive methods for defining relative points in time using a custom protocol. It supports chaining of datetime intervals in a parent-child fashion, abstracted with attributes and indexes. For example: year.month[2].

Note: This library is in its alpha stage. Contributions and feedback are welcome!

---

Features

Flexible Interval Expressions: Define points in time by chaining properties of the `Expression` object and indexing them.

# Second day of the 3rd week of the month, 0-based indexing
expr = Expression()
expr.month.week[2].day[1]

Negative Indexing: Support for negative indices to easily access "last" instances of time units (e.g., the last day of the month).

expr.month.day[-1] # Last day of the month

Rollover Handling: Control whether units roll over into the next larger unit or strictly validate.

By default, invalid indices roll over into the parent. For example:

nonleap_date = datetime(2021, 1, 1)
feb_29_expr = Expression().year.month[1].day[28]
feb_29_2021 = feb_29_expr(nonleap_date)
print(feb_29_2021)

2024-03-01 00:00:00

Similarly, with mathmatical operations:

expr = Expression()
feb_29_expr = expr.year.month[1].day[27] + expr.day.n(1)
feb_29_2021 = feb_29_expr(nonleap_date)
print(feb_29_2021)

2024-03-01 00:00:00

To disable this behavior, pass rollover=False:

feb_29_2021 = feb_29_expr(nonleap_date, rollover=False)

IndexError: Rollover occurred for Month from 2 to 3.

If you want operations to roll over, but not invalid indices, you can pass operation_safe=True:

feb_29_expr = expr.year.month[1].day[27] + expr.day.n(1)
feb_29_2021 = feb_29_expr(nonleap_date, rollover=False, operation_safe=True)
print(feb_29_2021)

2021-03-01 00:00:00

More Time Intervals: Provides proxies for additional units of time, such as centisecond, decisecond, and decade.

print(
  expr  
  .decade
  .year[0]
  .quarter[0]
  .month[0]
  .week[0]
  .day[0]
  .hour[0]
  .minute[0]
  .second[0]
  .decisecond[0]
  .millisecond[0]
  .microsecond[0]
)

Decade > Year[1] > Quarter[1] > Month[1] > Week[1] > Day[1] > Hour[1] > Minute[1] > Second[1] > Decisecond[1] > Millisecond[1] > Microsecond[1]

Lazy Evaluation: Define relative time objects and evaluate them against a specific `datetime` object later.

# Last day of the month
last_day_of_month = expr.month.day[-1]

# Evaluates after recieving a baseline
last_day_this_month = last_month_day(datime.now())
print(last_day_this_month)

2024-09-30 00:00:00

Lazy Chain Validation: Rough validation is provided during chaining and indexing, but full validation occurs only during evaluation.

expr.month.day[99]

----------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[4], line 1
----> 1 expr.month.day[99]

ValueError: Day cannot accept index 99 of Month (max: 34)

Lazy Arithmetic Evaluation: Add or subtract relative deltas from an `Expression`. The library ensures proper order of operations during evaluation.

from pyinterval.expression import Expression
from datetime import datetime

expr = Expression()            # Instantiate the Expression object
date_expr = expr.year.month[1] # Start with the year and second month
date_expr += expr.day.n(1)     # Add 1 day to the expression
date_expr = date_expr.day[27]  # Set the 28th day (0-based, so index 27)
date_expr -= expr.month.n(1)   # Subtract one month from the expression

time_expr = (                  # Set the time to 01:01:01.101001
    date_expr
    .hour[0]
    .minute[0]
    .second[0]
    .decisecond[0]
    .millisecond[0]
    .microsecond[0]
)

final_expr = time_expr + expr.month.n(1)  # Add one month to the final result

result = final_expr(datetime(2021, 1 ,1)) # Evaluate the expression with a non-leap year
print(result)

result = final_expr(datetime(2024, 1 ,1)) # Evaluate the expression with a leap year
print(result)

2021-03-01 01:01:01.101001
2024-02-29 01:01:01.101001

Self-Explanatory String Representations: The `__repr__` method produces an effective representation of the relative time expression.

print(some_date)

Year > Month[3] > Day[1] + relativedelta(months=-1, days=-1) > Hour[12] > Minute[45]

High Performance: Complex relative dates are evaluated quickly and efficiently.

%%timeit

expr = Expression()
date_expr = expr.year.month[-11]
date_expr += expr.day.n(59)
date_expr -= expr.month.n(1634)

time_expr = (
    date_expr
    .hour[-823]
    .minute[-59]
    .second[-59]
    .decisecond[-9]
    .millisecond[-99]
    .microsecond[-999]
)

final_expr = time_expr + expr.month.n(1)

result1 = final_expr(datetime(2021, 1 ,1)) # Evaluate the expression with a non-leap year
result2 = final_expr(datetime(2024, 1 ,1)) # Evaluate the expression with a leap year
assert result1 != result2

385 μs ± 4.22 μs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

---

Interval Expression Rules

You can chain intervals together to describe increasingly granular time ranges. For example:

expr = Expression().year.month[2].day[4]  # 5th day of the 3rd month (0-based indexing)

The structure consists of 4 main parts: root, root scope, scope units, and indices:

Root().root_scope.scope_unit[index]

• Root: The Expression() object is always the root, encapsulating the logic.
• Root Scope: Defines the top-level time unit (e.g., year).
• Scope Unit: Represents smaller time units (e.g., month, day).
• Index: Represents the position within the parent element (e.g., year.month[2] refers to the 3rd month of the year).

The root scope cannot be indexed directly. However, you can call a root scope's n attribute (e.g., Expression().quarter.n(1)) to generate a timedelta.

print(expr.quarter.n(2))  # 6 months relative to the current date

relativedelta(months=+6)

You can pass a date to a scope unit to evaluate the expression and create an absolute datetime.

relative_date = expr.year.quarter[-1].month[1].week[-1].day[5]
absolute_date = relative_date(datetime.now())
print(absolute_date)

2024-11-26 00:00:00

---

Roadmap

• Smart Indexing: Implement slicing ([start:stop:step]) to generate date ranges at specified granularity.
• Validation: Add expression.verify(datetime) -> bool to check if a datetime matches the expression or falls within the specified range.
• Timedelta Compatibility: Allow arithmetic with timedelta objects on expression chains (e.g., chain - timedelta(days=1)).
• Dynamic Chain Building: Provide built-in methods to dynamically create chains, reducing manual effort for constructing expressions.
• Chain Operators: Support chaining with the + operator and breaking chains with the - operator.
• Advanced Smart Indexing: Allow [start:stop] to be expressions and [:step] to be a timedelta.
• Rust Implementation: Rebuild the library in Rust, with Python bindings.

---