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Luciano Ramalho
4
18-with-match/README.rst
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4
18-with-match/README.rst
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@@ -0,0 +1,4 @@
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Sample code for Chapter 15 - "Context managers and something else"
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From the book "Fluent Python" by Luciano Ramalho (O'Reilly, 2015)
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http://shop.oreilly.com/product/0636920032519.do
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109
18-with-match/lisplus/examples_test.py
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109
18-with-match/lisplus/examples_test.py
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"""
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Doctests for `parse`:
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>>> from lis import parse
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# tag::PARSE_DEMO[]
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>>> parse('1.5') # <1>
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1.5
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>>> parse('set!') # <2>
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'set!'
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>>> parse('(gcd 18 44)') # <3>
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['gcd', 18, 44]
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>>> parse('(- m (* n (// m n)))') # <4>
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['-', 'm', ['*', 'n', ['//', 'm', 'n']]]
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# end::PARSE_DEMO[]
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"""
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import math
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from lis import run
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fact_src = """
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(define (! n)
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(if (< n 2)
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1
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(* n (! (- n 1)))
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)
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)
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(! 42)
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"""
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def test_factorial():
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got = run(fact_src)
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assert got == 1405006117752879898543142606244511569936384000000000
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assert got == math.factorial(42)
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gcd_src = """
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(define (mod m n)
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(- m (* n (// m n))))
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(define (gcd m n)
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(if (= n 0)
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m
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(gcd n (mod m n))))
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(gcd 18 45)
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"""
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def test_gcd():
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got = run(gcd_src)
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assert got == 9
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quicksort_src = """
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(define (quicksort lst)
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(if (null? lst)
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lst
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(begin
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(define pivot (car lst))
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(define rest (cdr lst))
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(append
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(quicksort
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(filter (lambda (x) (< x pivot)) rest))
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(list pivot)
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(quicksort
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(filter (lambda (x) (>= x pivot)) rest)))
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)
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)
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)
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(quicksort (list 2 1 6 3 4 0 8 9 7 5))
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"""
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def test_quicksort():
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got = run(quicksort_src)
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assert got == [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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# Example from Structure and Interpretation of Computer Programs
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# https://mitpress.mit.edu/sites/default/files/sicp/full-text/sicp/book/node12.html
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newton_src = """
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(define (sqrt x)
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(sqrt-iter 1.0 x))
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(define (sqrt-iter guess x)
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(if (good-enough? guess x)
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guess
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(sqrt-iter (improve guess x) x)))
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(define (good-enough? guess x)
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(< (abs (- (* guess guess) x)) 0.001))
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(define (improve guess x)
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(average guess (/ x guess)))
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(define (average x y)
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(/ (+ x y) 2))
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(sqrt 123454321)
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"""
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def test_newton():
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got = run(newton_src)
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assert math.isclose(got, 11111)
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closure_src = """
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(define (make-adder increment)
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(lambda (x) (+ increment x))
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)
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(define inc (make-adder 1))
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(inc 99)
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"""
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def test_newton():
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got = run(closure_src)
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assert got == 100
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192
18-with-match/lisplus/lis.py
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192
18-with-match/lisplus/lis.py
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################ Lispy: Scheme Interpreter in Python 3.10
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## (c) Peter Norvig, 2010-18; See http://norvig.com/lispy.html
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## Minor edits for Fluent Python, Second Edition (O'Reilly, 2021)
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## by Luciano Ramalho, adding type hints and pattern matching.
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################ imports and types
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import math
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import operator as op
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from collections import ChainMap
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from collections.abc import MutableMapping, Iterator
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from itertools import chain
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from typing import Any, TypeAlias
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Symbol: TypeAlias = str
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Atom: TypeAlias = float | int | Symbol
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Expression: TypeAlias = Atom | list
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Environment: TypeAlias = MutableMapping[Symbol, object]
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class Procedure:
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"A user-defined Scheme procedure."
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def __init__(self, parms: list[Symbol], body: list[Expression], env: Environment):
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self.parms = parms
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self.body = body
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self.env = env
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def __call__(self, *args: Expression) -> Any:
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local_env = dict(zip(self.parms, args))
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env: Environment = ChainMap(local_env, self.env)
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for exp in self.body:
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result = evaluate(exp, env)
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return result
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################ global environment
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def standard_env() -> Environment:
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"An environment with some Scheme standard procedures."
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env: Environment = {}
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env.update(vars(math)) # sin, cos, sqrt, pi, ...
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env.update({
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'+': op.add,
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'-': op.sub,
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'*': op.mul,
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'/': op.truediv,
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'//': op.floordiv,
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'>': op.gt,
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'<': op.lt,
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'>=': op.ge,
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'<=': op.le,
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'=': op.eq,
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'abs': abs,
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'append': lambda *args: list(chain(*args)),
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'apply': lambda proc, args: proc(*args),
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'begin': lambda *x: x[-1],
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'car': lambda x: x[0],
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'cdr': lambda x: x[1:],
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'cons': lambda x, y: [x] + y,
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'eq?': op.is_,
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'equal?': op.eq,
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'filter': lambda *args: list(filter(*args)),
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'length': len,
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'list': lambda *x: list(x),
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'list?': lambda x: isinstance(x, list),
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'map': lambda *args: list(map(*args)),
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'max': max,
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'min': min,
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'not': op.not_,
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'null?': lambda x: x == [],
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'number?': lambda x: isinstance(x, (int, float)),
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'procedure?': callable,
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'round': round,
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'symbol?': lambda x: isinstance(x, Symbol),
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})
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return env
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################ parse, tokenize, and read_from_tokens
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def parse(program: str) -> Expression:
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"Read a Scheme expression from a string."
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return read_from_tokens(tokenize(program))
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def tokenize(s: str) -> list[str]:
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"Convert a string into a list of tokens."
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return s.replace('(', ' ( ').replace(')', ' ) ').split()
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def read_from_tokens(tokens: list[str]) -> Expression:
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"Read an expression from a sequence of tokens."
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if len(tokens) == 0:
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raise SyntaxError('unexpected EOF while reading')
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token = tokens.pop(0)
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if '(' == token:
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exp = []
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while tokens[0] != ')':
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exp.append(read_from_tokens(tokens))
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tokens.pop(0) # discard ')'
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return exp
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elif ')' == token:
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raise SyntaxError('unexpected )')
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else:
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return parse_atom(token)
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def parse_atom(token: str) -> Atom:
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"Numbers become numbers; every other token is a symbol."
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try:
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return int(token)
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except ValueError:
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try:
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return float(token)
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except ValueError:
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return Symbol(token)
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################ interaction: a REPL
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def repl(prompt: str = 'lis.py> ') -> None:
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"A prompt-read-evaluate-print loop."
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global_env: Environment = standard_env()
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while True:
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val = evaluate(parse(input(prompt)), global_env)
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if val is not None:
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print(lispstr(val))
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def lispstr(exp: object) -> str:
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"Convert a Python object back into a Lisp-readable string."
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if isinstance(exp, list):
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return '(' + ' '.join(map(lispstr, exp)) + ')'
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else:
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return str(exp)
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################ eval
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# tag::EVALUATE[]
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def evaluate(exp: Expression, env: Environment) -> Any:
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"Evaluate an expression in an environment."
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match exp:
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case int(x) | float(x):
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return x
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case Symbol(var):
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return env[var]
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case []:
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return []
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case ['quote', exp]:
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return exp
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case ['if', test, consequence, alternative]:
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if evaluate(test, env):
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return evaluate(consequence, env)
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else:
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return evaluate(alternative, env)
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case ['define', Symbol(var), value_exp]:
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env[var] = evaluate(value_exp, env)
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case ['define', [Symbol(name), *parms], *body]:
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env[name] = Procedure(parms, body, env)
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case ['lambda', [*parms], *body]:
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return Procedure(parms, body, env)
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case [op, *args]:
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proc = evaluate(op, env)
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values = [evaluate(arg, env) for arg in args]
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return proc(*values)
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case _:
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raise SyntaxError(repr(exp))
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# end::EVALUATE[]
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################ non-interactive execution
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def run_lines(source: str) -> Iterator[Any]:
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global_env: Environment = standard_env()
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tokens = tokenize(source)
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while tokens:
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exp = read_from_tokens(tokens)
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yield evaluate(exp, global_env)
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def run(source: str) -> Any:
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for result in run_lines(source):
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pass
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return result
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182
18-with-match/lisplus/lis_test.py
Normal file
182
18-with-match/lisplus/lis_test.py
Normal file
@@ -0,0 +1,182 @@
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from typing import Optional
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from pytest import mark, fixture
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from lis import parse, evaluate, Expression, Environment, standard_env
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############################################################# tests for parse
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@mark.parametrize( 'source, expected', [
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('7', 7),
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('x', 'x'),
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('(sum 1 2 3)', ['sum', 1, 2, 3]),
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('(+ (* 2 100) (* 1 10))', ['+', ['*', 2, 100], ['*', 1, 10]]),
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('99 100', 99), # parse stops at the first complete expression
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('(a)(b)', ['a']),
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])
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def test_parse(source: str, expected: Expression) -> None:
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got = parse(source)
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assert got == expected
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|
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|
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########################################################## tests for evaluate
|
||||
|
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# Norvig's tests are not isolated: they assume the
|
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# same environment from first to last test.
|
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global_env_for_first_test = standard_env()
|
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|
||||
@mark.parametrize( 'source, expected', [
|
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("(quote (testing 1 (2.0) -3.14e159))", ['testing', 1, [2.0], -3.14e159]),
|
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("(+ 2 2)", 4),
|
||||
("(+ (* 2 100) (* 1 10))", 210),
|
||||
("(if (> 6 5) (+ 1 1) (+ 2 2))", 2),
|
||||
("(if (< 6 5) (+ 1 1) (+ 2 2))", 4),
|
||||
("(define x 3)", None),
|
||||
("x", 3),
|
||||
("(+ x x)", 6),
|
||||
("((lambda (x) (+ x x)) 5)", 10),
|
||||
("(define twice (lambda (x) (* 2 x)))", None),
|
||||
("(twice 5)", 10),
|
||||
("(define compose (lambda (f g) (lambda (x) (f (g x)))))", None),
|
||||
("((compose list twice) 5)", [10]),
|
||||
("(define repeat (lambda (f) (compose f f)))", None),
|
||||
("((repeat twice) 5)", 20),
|
||||
("((repeat (repeat twice)) 5)", 80),
|
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("(define fact (lambda (n) (if (<= n 1) 1 (* n (fact (- n 1))))))", None),
|
||||
("(fact 3)", 6),
|
||||
("(fact 50)", 30414093201713378043612608166064768844377641568960512000000000000),
|
||||
("(define abs (lambda (n) ((if (> n 0) + -) 0 n)))", None),
|
||||
("(list (abs -3) (abs 0) (abs 3))", [3, 0, 3]),
|
||||
("""(define combine (lambda (f)
|
||||
(lambda (x y)
|
||||
(if (null? x) (quote ())
|
||||
(f (list (car x) (car y))
|
||||
((combine f) (cdr x) (cdr y)))))))""", None),
|
||||
("(define zip (combine cons))", None),
|
||||
("(zip (list 1 2 3 4) (list 5 6 7 8))", [[1, 5], [2, 6], [3, 7], [4, 8]]),
|
||||
("""(define riff-shuffle (lambda (deck)
|
||||
(begin
|
||||
(define take (lambda (n seq) (if (<= n 0) (quote ()) (cons (car seq) (take (- n 1) (cdr seq))))))
|
||||
(define drop (lambda (n seq) (if (<= n 0) seq (drop (- n 1) (cdr seq)))))
|
||||
(define mid (lambda (seq) (/ (length seq) 2)))
|
||||
((combine append) (take (mid deck) deck) (drop (mid deck) deck)))))""", None),
|
||||
("(riff-shuffle (list 1 2 3 4 5 6 7 8))", [1, 5, 2, 6, 3, 7, 4, 8]),
|
||||
("((repeat riff-shuffle) (list 1 2 3 4 5 6 7 8))", [1, 3, 5, 7, 2, 4, 6, 8]),
|
||||
("(riff-shuffle (riff-shuffle (riff-shuffle (list 1 2 3 4 5 6 7 8))))", [1,2,3,4,5,6,7,8]),
|
||||
])
|
||||
def test_evaluate(source: str, expected: Optional[Expression]) -> None:
|
||||
got = evaluate(parse(source), global_env_for_first_test)
|
||||
assert got == expected
|
||||
|
||||
|
||||
@fixture
|
||||
def std_env() -> Environment:
|
||||
return standard_env()
|
||||
|
||||
# tests for each of the cases in evaluate
|
||||
|
||||
def test_evaluate_variable() -> None:
|
||||
env: Environment = dict(x=10)
|
||||
source = 'x'
|
||||
expected = 10
|
||||
got = evaluate(parse(source), env)
|
||||
assert got == expected
|
||||
|
||||
|
||||
def test_evaluate_literal(std_env: Environment) -> None:
|
||||
source = '3.3'
|
||||
expected = 3.3
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got == expected
|
||||
|
||||
|
||||
def test_evaluate_quote(std_env: Environment) -> None:
|
||||
source = '(quote (1.1 is not 1))'
|
||||
expected = [1.1, 'is', 'not', 1]
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got == expected
|
||||
|
||||
|
||||
def test_evaluate_if_true(std_env: Environment) -> None:
|
||||
source = '(if 1 10 no-such-thing)'
|
||||
expected = 10
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got == expected
|
||||
|
||||
|
||||
def test_evaluate_if_false(std_env: Environment) -> None:
|
||||
source = '(if 0 no-such-thing 20)'
|
||||
expected = 20
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got == expected
|
||||
|
||||
|
||||
def test_define(std_env: Environment) -> None:
|
||||
source = '(define answer (* 6 7))'
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got is None
|
||||
assert std_env['answer'] == 42
|
||||
|
||||
|
||||
def test_lambda(std_env: Environment) -> None:
|
||||
source = '(lambda (a b) (if (>= a b) a b))'
|
||||
func = evaluate(parse(source), std_env)
|
||||
assert func.parms == ['a', 'b']
|
||||
assert func.body == [['if', ['>=', 'a', 'b'], 'a', 'b']]
|
||||
assert func.env is std_env
|
||||
assert func(1, 2) == 2
|
||||
assert func(3, 2) == 3
|
||||
|
||||
|
||||
def test_begin(std_env: Environment) -> None:
|
||||
source = """
|
||||
(begin
|
||||
(define x (* 2 3))
|
||||
(* x 7)
|
||||
)
|
||||
"""
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got == 42
|
||||
|
||||
|
||||
def test_invocation_builtin_car(std_env: Environment) -> None:
|
||||
source = '(car (quote (11 22 33)))'
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got == 11
|
||||
|
||||
|
||||
def test_invocation_builtin_append(std_env: Environment) -> None:
|
||||
source = '(append (quote (a b)) (quote (c d)))'
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got == ['a', 'b', 'c', 'd']
|
||||
|
||||
|
||||
def test_invocation_builtin_map(std_env: Environment) -> None:
|
||||
source = '(map (lambda (x) (* x 2)) (quote (1 2 3))))'
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got == [2, 4, 6]
|
||||
|
||||
|
||||
def test_invocation_user_procedure(std_env: Environment) -> None:
|
||||
source = """
|
||||
(begin
|
||||
(define max (lambda (a b) (if (>= a b) a b)))
|
||||
(max 22 11)
|
||||
)
|
||||
"""
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got == 22
|
||||
|
||||
|
||||
###################################### for py3.10/lis.py only
|
||||
|
||||
def test_define_function(std_env: Environment) -> None:
|
||||
source = '(define (max a b) (if (>= a b) a b))'
|
||||
got = evaluate(parse(source), std_env)
|
||||
assert got is None
|
||||
max_fn = std_env['max']
|
||||
assert max_fn.parms == ['a', 'b']
|
||||
assert max_fn.body == [['if', ['>=', 'a', 'b'], 'a', 'b']]
|
||||
assert max_fn.env is std_env
|
||||
assert max_fn(1, 2) == 2
|
||||
assert max_fn(3, 2) == 3
|
||||
64
18-with-match/lisplus/meta_test.py
Normal file
64
18-with-match/lisplus/meta_test.py
Normal file
@@ -0,0 +1,64 @@
|
||||
import operator as op
|
||||
|
||||
from lis import run
|
||||
|
||||
env_scm = """
|
||||
(define standard-env (list
|
||||
(list (quote +) +)
|
||||
(list (quote -) -)
|
||||
))
|
||||
standard-env
|
||||
"""
|
||||
|
||||
def test_env_build():
|
||||
got = run(env_scm)
|
||||
assert got == [['+', op.add], ['-', op.sub]]
|
||||
|
||||
scan_scm = """
|
||||
(define l (quote (a b c)))
|
||||
(define (scan what where)
|
||||
(if (null? where)
|
||||
()
|
||||
(if (eq? what (car where))
|
||||
what
|
||||
(scan what (cdr where))))
|
||||
)
|
||||
"""
|
||||
|
||||
def test_scan():
|
||||
source = scan_scm + '(scan (quote a) l )'
|
||||
got = run(source)
|
||||
assert got == 'a'
|
||||
|
||||
|
||||
def test_scan_not_found():
|
||||
source = scan_scm + '(scan (quote z) l )'
|
||||
got = run(source)
|
||||
assert got == []
|
||||
|
||||
|
||||
lookup_scm = """
|
||||
(define env (list
|
||||
(list (quote +) +)
|
||||
(list (quote -) -)
|
||||
))
|
||||
(define (lookup what where)
|
||||
(if (null? where)
|
||||
()
|
||||
(if (eq? what (car (car where)))
|
||||
(car (cdr (car where)))
|
||||
(lookup what (cdr where))))
|
||||
)
|
||||
"""
|
||||
|
||||
def test_lookup():
|
||||
source = lookup_scm + '(lookup (quote +) env)'
|
||||
got = run(source)
|
||||
assert got == op.add
|
||||
|
||||
|
||||
def test_lookup_not_found():
|
||||
source = lookup_scm + '(lookup (quote z) env )'
|
||||
got = run(source)
|
||||
assert got == []
|
||||
|
||||
92
18-with-match/mirror.py
Normal file
92
18-with-match/mirror.py
Normal file
@@ -0,0 +1,92 @@
|
||||
"""
|
||||
A "mirroring" ``stdout`` context.
|
||||
|
||||
While active, the context manager reverses text output to
|
||||
``stdout``::
|
||||
|
||||
# tag::MIRROR_DEMO_1[]
|
||||
|
||||
>>> from mirror import LookingGlass
|
||||
>>> with LookingGlass() as what: # <1>
|
||||
... print('Alice, Kitty and Snowdrop') # <2>
|
||||
... print(what)
|
||||
...
|
||||
pordwonS dna yttiK ,ecilA # <3>
|
||||
YKCOWREBBAJ
|
||||
>>> what # <4>
|
||||
'JABBERWOCKY'
|
||||
>>> print('Back to normal.') # <5>
|
||||
Back to normal.
|
||||
|
||||
# end::MIRROR_DEMO_1[]
|
||||
|
||||
|
||||
This exposes the context manager operation::
|
||||
|
||||
# tag::MIRROR_DEMO_2[]
|
||||
|
||||
>>> from mirror import LookingGlass
|
||||
>>> manager = LookingGlass() # <1>
|
||||
>>> manager
|
||||
<mirror.LookingGlass object at 0x2a578ac>
|
||||
>>> monster = manager.__enter__() # <2>
|
||||
>>> monster == 'JABBERWOCKY' # <3>
|
||||
eurT
|
||||
>>> monster
|
||||
'YKCOWREBBAJ'
|
||||
>>> manager
|
||||
>ca875a2x0 ta tcejbo ssalGgnikooL.rorrim<
|
||||
>>> manager.__exit__(None, None, None) # <4>
|
||||
>>> monster
|
||||
'JABBERWOCKY'
|
||||
|
||||
# end::MIRROR_DEMO_2[]
|
||||
|
||||
The context manager can handle and "swallow" exceptions.
|
||||
|
||||
# tag::MIRROR_DEMO_3[]
|
||||
|
||||
>>> from mirror import LookingGlass
|
||||
>>> with LookingGlass():
|
||||
... print('Humpty Dumpty')
|
||||
... x = 1/0 # <1>
|
||||
... print('END') # <2>
|
||||
...
|
||||
ytpmuD ytpmuH
|
||||
Please DO NOT divide by zero!
|
||||
>>> with LookingGlass():
|
||||
... print('Humpty Dumpty')
|
||||
... x = no_such_name # <1>
|
||||
... print('END') # <2>
|
||||
...
|
||||
Traceback (most recent call last):
|
||||
...
|
||||
NameError: name 'no_such_name' is not defined
|
||||
|
||||
# end::MIRROR_DEMO_3[]
|
||||
|
||||
"""
|
||||
|
||||
|
||||
# tag::MIRROR_EX[]
|
||||
class LookingGlass:
|
||||
|
||||
def __enter__(self): # <1>
|
||||
import sys
|
||||
self.original_write = sys.stdout.write # <2>
|
||||
sys.stdout.write = self.reverse_write # <3>
|
||||
return 'JABBERWOCKY' # <4>
|
||||
|
||||
def reverse_write(self, text): # <5>
|
||||
self.original_write(text[::-1])
|
||||
|
||||
def __exit__(self, exc_type, exc_value, traceback): # <6>
|
||||
import sys # <7>
|
||||
sys.stdout.write = self.original_write # <8>
|
||||
if exc_type is ZeroDivisionError: # <9>
|
||||
print('Please DO NOT divide by zero!')
|
||||
return True # <10>
|
||||
# <11>
|
||||
|
||||
|
||||
# end::MIRROR_EX[]
|
||||
64
18-with-match/mirror_gen.py
Normal file
64
18-with-match/mirror_gen.py
Normal file
@@ -0,0 +1,64 @@
|
||||
"""
|
||||
A "mirroring" ``stdout`` context manager.
|
||||
|
||||
While active, the context manager reverses text output to
|
||||
``stdout``::
|
||||
|
||||
# tag::MIRROR_GEN_DEMO_1[]
|
||||
|
||||
>>> from mirror_gen import looking_glass
|
||||
>>> with looking_glass() as what: # <1>
|
||||
... print('Alice, Kitty and Snowdrop')
|
||||
... print(what)
|
||||
...
|
||||
pordwonS dna yttiK ,ecilA
|
||||
YKCOWREBBAJ
|
||||
>>> what
|
||||
'JABBERWOCKY'
|
||||
|
||||
# end::MIRROR_GEN_DEMO_1[]
|
||||
|
||||
|
||||
This exposes the context manager operation::
|
||||
|
||||
# tag::MIRROR_GEN_DEMO_2[]
|
||||
|
||||
>>> from mirror_gen import looking_glass
|
||||
>>> manager = looking_glass() # <1>
|
||||
>>> manager # doctest: +ELLIPSIS
|
||||
<contextlib._GeneratorContextManager object at 0x...>
|
||||
>>> monster = manager.__enter__() # <2>
|
||||
>>> monster == 'JABBERWOCKY' # <3>
|
||||
eurT
|
||||
>>> monster
|
||||
'YKCOWREBBAJ'
|
||||
>>> manager # doctest: +ELLIPSIS
|
||||
>...x0 ta tcejbo reganaMtxetnoCrotareneG_.biltxetnoc<
|
||||
>>> manager.__exit__(None, None, None) # <4>
|
||||
>>> monster
|
||||
'JABBERWOCKY'
|
||||
|
||||
# end::MIRROR_GEN_DEMO_2[]
|
||||
|
||||
"""
|
||||
|
||||
|
||||
# tag::MIRROR_GEN_EX[]
|
||||
|
||||
import contextlib
|
||||
|
||||
|
||||
@contextlib.contextmanager # <1>
|
||||
def looking_glass():
|
||||
import sys
|
||||
original_write = sys.stdout.write # <2>
|
||||
|
||||
def reverse_write(text): # <3>
|
||||
original_write(text[::-1])
|
||||
|
||||
sys.stdout.write = reverse_write # <4>
|
||||
yield 'JABBERWOCKY' # <5>
|
||||
sys.stdout.write = original_write # <6>
|
||||
|
||||
|
||||
# end::MIRROR_GEN_EX[]
|
||||
101
18-with-match/mirror_gen_exc.py
Normal file
101
18-with-match/mirror_gen_exc.py
Normal file
@@ -0,0 +1,101 @@
|
||||
"""
|
||||
A "mirroring" ``stdout`` context manager.
|
||||
|
||||
While active, the context manager reverses text output to
|
||||
``stdout``::
|
||||
|
||||
# tag::MIRROR_GEN_DEMO_1[]
|
||||
|
||||
>>> from mirror_gen import looking_glass
|
||||
>>> with looking_glass() as what: # <1>
|
||||
... print('Alice, Kitty and Snowdrop')
|
||||
... print(what)
|
||||
...
|
||||
pordwonS dna yttiK ,ecilA
|
||||
YKCOWREBBAJ
|
||||
>>> what
|
||||
'JABBERWOCKY'
|
||||
|
||||
# end::MIRROR_GEN_DEMO_1[]
|
||||
|
||||
|
||||
This exposes the context manager operation::
|
||||
|
||||
# tag::MIRROR_GEN_DEMO_2[]
|
||||
|
||||
>>> from mirror_gen import looking_glass
|
||||
>>> manager = looking_glass() # <1>
|
||||
>>> manager # doctest: +ELLIPSIS
|
||||
<contextlib._GeneratorContextManager object at 0x...>
|
||||
>>> monster = manager.__enter__() # <2>
|
||||
>>> monster == 'JABBERWOCKY' # <3>
|
||||
eurT
|
||||
>>> monster
|
||||
'YKCOWREBBAJ'
|
||||
>>> manager # doctest: +ELLIPSIS
|
||||
>...x0 ta tcejbo reganaMtxetnoCrotareneG_.biltxetnoc<
|
||||
>>> manager.__exit__(None, None, None) # <4>
|
||||
>>> monster
|
||||
'JABBERWOCKY'
|
||||
|
||||
# end::MIRROR_GEN_DEMO_2[]
|
||||
|
||||
The context manager can handle and "swallow" exceptions.
|
||||
The following test does not pass under doctest (a
|
||||
ZeroDivisionError is reported by doctest) but passes
|
||||
if executed by hand in the Python 3 console (the exception
|
||||
is handled by the context manager):
|
||||
|
||||
# tag::MIRROR_GEN_DEMO_3[]
|
||||
|
||||
>>> from mirror_gen import looking_glass
|
||||
>>> with looking_glass():
|
||||
... print('Humpty Dumpty')
|
||||
... x = 1/0 # <1>
|
||||
... print('END') # <2>
|
||||
...
|
||||
ytpmuD ytpmuH
|
||||
Please DO NOT divide by zero!
|
||||
|
||||
# end::MIRROR_GEN_DEMO_3[]
|
||||
|
||||
>>> with looking_glass():
|
||||
... print('Humpty Dumpty')
|
||||
... x = no_such_name # <1>
|
||||
... print('END') # <2>
|
||||
...
|
||||
Traceback (most recent call last):
|
||||
...
|
||||
NameError: name 'no_such_name' is not defined
|
||||
|
||||
|
||||
|
||||
"""
|
||||
|
||||
|
||||
# tag::MIRROR_GEN_EXC[]
|
||||
|
||||
import contextlib
|
||||
|
||||
|
||||
@contextlib.contextmanager
|
||||
def looking_glass():
|
||||
import sys
|
||||
original_write = sys.stdout.write
|
||||
|
||||
def reverse_write(text):
|
||||
original_write(text[::-1])
|
||||
|
||||
sys.stdout.write = reverse_write
|
||||
msg = '' # <1>
|
||||
try:
|
||||
yield 'JABBERWOCKY'
|
||||
except ZeroDivisionError: # <2>
|
||||
msg = 'Please DO NOT divide by zero!'
|
||||
finally:
|
||||
sys.stdout.write = original_write # <3>
|
||||
if msg:
|
||||
print(msg) # <4>
|
||||
|
||||
|
||||
# end::MIRROR_GEN_EXC[]
|
||||
Reference in New Issue
Block a user