115 lines
3.1 KiB
Python
115 lines
3.1 KiB
Python
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from typing import List, Optional, TypeVar
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T = TypeVar("T")
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class Stack(List[T]):
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_max_length: Optional[int]
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def __init__(self, *args, max_length: Optional[int] = None):
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initial_entries = args
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if max_length:
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initial_entries = initial_entries[:max_length]
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super().__init__(initial_entries)
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self._max_length = max_length
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def empty(self) -> bool:
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"""Tests if this stack is empty."""
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return len(self) == 0
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def peek(self) -> Optional[T]:
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"""Looks at the object at the top (last/most recently added) of this
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stack without removing it from the stack."""
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return self.at(-1)
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def pop(self) -> Optional[T]:
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"""Removes the object at the top of this stack and returns that object
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as the value of this function."""
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try:
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value = super().pop()
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return value
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except IndexError:
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pass
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def push(self, item: T) -> None:
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"""Pushes an item onto the top of this stack.
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Proxy of List.append
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Limits Stack Length to _max_length entries
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"""
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self.append(item)
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if self._max_length:
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del self[: -self._max_length]
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def search(self, x: T) -> Optional[int]:
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"""Returns the 0-based position of the last item whose value is equal
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to x on this stack.
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We deviate from the typical 1-based position used by Stack
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classes (i.e. Java) because most python users (and developers in
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general) are accustomed to 0-based indexing.
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"""
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copy = self.copy()
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copy.reverse()
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try:
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found_index = copy.index(x)
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return len(self) - found_index - 1
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except ValueError:
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pass
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def at(self, index: int, default: Optional[T] = None) -> Optional[T]:
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"""Returns the item located at the index.
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If the index does not exist in the stack (Overflow or
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Underflow), None is returned instead.
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"""
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try:
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value = self[index]
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return value
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except IndexError:
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return default
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def copy(self) -> "Stack[T]":
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"""Returns a copy of the current Stack."""
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copy = super().copy()
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return Stack(*copy)
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@property
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def first(self) -> Optional[T]:
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"""Returns the first item of the stack without removing it.
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Same as Stack.bottom.
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"""
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return self.at(0)
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@property
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def last(self) -> Optional[T]:
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"""Returns the last item of the stack without removing it.
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Same as Stack.top.
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"""
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return self.at(-1)
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@property
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def bottom(self) -> Optional[T]:
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"""Returns the item on the bottom of the stack without removing it.
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Same as Stack.first.
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"""
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return self.at(0)
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@property
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def top(self) -> Optional[T]:
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"""Returns the item on the top of the stack without removing it.
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Same as Stack.last.
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"""
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return self.at(-1)
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@property
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def length(self) -> int:
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"""Returns the number of items in the Stack."""
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return len(self)
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