In-Memory Database with TTL

Background. An in-memory store holds keys. Each key holds named fields, and each field holds an integer value. Every call is told the time at which it is made.

Implement the class FieldStore in store.py, with the methods put, read, replace_if, remove_if and list_fields.

• put(now, key, field, value) stores value in field of key, creating the key and the field as needed and replacing any value the field held.
• read(now, key, field) returns the value in field of key, or None when that key or field holds no value.
• replace_if(now, key, field, expected, value): if the field holds exactly expected, stores value in it and returns True. If the field holds no value or a different one, returns False and changes nothing.
• remove_if(now, key, field, expected): if the field holds exactly expected, removes the field and returns True. If it holds no value or a different one, returns False and changes nothing.
• list_fields(now, key, prefix="") returns the fields of key whose names start with prefix, as a list of (field, value) tuples sorted by field name in Python's string order. The default prefix="" lists every field; a key with no fields lists [].

now is the time of the call: an integer from 0 up to 10^15 that is larger on every call than on the one before. The answers here do not depend on it. Only put, a successful replace_if and a successful remove_if change the store.

Example

s = FieldStore()
s.put(1, "lamp", "watts", 40)
s.put(2, "lamp", "hours", 3)
s.put(3, "lamp", "hue", 7)
s.read(4, "lamp", "watts")                # 40
s.replace_if(5, "lamp", "watts", 60, 75)  # False: it holds 40
s.replace_if(6, "lamp", "watts", 40, 75)  # True
s.remove_if(7, "lamp", "hours", 3)        # True
s.list_fields(8, "lamp")                  # [("hue", 7), ("watts", 75)]
s.list_fields(9, "lamp", "h")             # [("hue", 7)]
s.read(10, "desk", "watts")               # None

Notes

• Keys and fields are non-empty strings, prefix is any string, and values and expected are any integers. Inputs are always valid.
• All 13 tests in tests/test_part1.py fail on the starter.

Done when Submit goes green.