Question

如何实现像python中的defer语句一样的东西？

Defer将函数调用推送到堆栈。当包含defer语句的函数返回时，在defer语句首先位于内部的范围内，逐个弹出并执行deferred函数调用。延迟语句看起来像函数调用，但在弹出之前不会执行。

去举例说明它是如何运作的：

func main() {
    fmt.Println("counting")

    var a *int
    for i := 0; i < 10; i++ {
        a = &i
        defer fmt.Println(*a, i)
    }

    x := 42
    a = &x

    fmt.Println("done")
}

输出：

counting
done
9 9
8 8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
0 0

去用例的例子：

var m sync.Mutex
func someFunction() {
    m.Lock()
    defer m.Unlock()
    // Whatever you want, with as many return statements as you want, wherever.
    // Simply forget that you ever locked a mutex, or that you have to remember to release it again.
}

Answer 1

要模仿defer fmt.Println(*a, i)示例，您可以use contextlib.ExitStack：

#!/usr/bin/env python3
from contextlib import ExitStack
from functools import partial

print("counting")
with ExitStack() as stack:
    for i in range(10):
        a = i
        stack.callback(partial(print, a, i))

    x = 42
    a = x
    print("done")

Output

counting
done
9 9
8 8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
0 0

很容易模仿互斥锁的情况：

def some_function(lock=Lock()):
    with lock:
        # whatever

Answer 2

我做了一个there（与2.x兼容）：

@defers_collector
def func():
    f = open('file.txt', 'w')
    defer(lambda: f.close())

    defer(lambda : print("Defer called!"))

    def my_defer():
    recover()

    defer(lambda: my_defer())

    print("Ok )")
    panic("WTF?")

    print("Never printed (((")


func()
print("Recovered!")

defers_collector的来源是：

# Go-style error handling

import inspect
import sys

def panic(x):
    raise Exception(x)

def defer(x):
    for f in inspect.stack():
    if '__defers__' in f[0].f_locals:
        f[0].f_locals['__defers__'].append(x)
        break

def recover():
    val = None
    for f in inspect.stack():
    loc = f[0].f_locals
    if f[3] == '__exit__' and '__suppress__' in loc:
        val = loc['exc_value']
        loc['__suppress__'].append(True)
        break
    return val

class DefersContainer(object):
    def __init__(self):
    # List for sustain refer in shallow clone
    self.defers = []

    def append(self, defer):
    self.defers.append(defer)

    def __enter__(self):
    pass

    def __exit__(self, exc_type, exc_value, traceback):
    __suppress__ = []
    for d in reversed(self.defers):
        try:
            d()
        except:
            __suppress__ = []
            exc_type, exc_value, traceback = sys.exc_info()
    return __suppress__


def defers_collector(func):
    def __wrap__(*args, **kwargs):
    __defers__ = DefersContainer()
    with __defers__:
        func(*args, **kwargs)
    return __wrap__

Answer 3

Python with statement与Go推迟的目的相似。

Python中的类似代码是：

mutex = Lock()

def someFunction():
    with mutex:
        # Whatever you want, with as many return statements
        # as you want, wherever. Simply forget that you ever
        # locked a mutex, or that you have to remember to 
        # release it again.

Answer 4

部分受@DenisKolodin answer启发的 defer 实现可作为pygolang，2的一部分使用：

   wc = wcfs.join(zurl)    │     wc = wcfs.join(zurl)
   defer(wc.close)         │     try:
                           │        ...
   ...                     │        ...
   ...                     │        ...
   ...                     │     finally:
                           │        wc.close()

Answer 5

在装饰器的帮助下，对jfs' answer的这种补充使ExitStack的想法更加深入：

@with_exit_stack
def counting(n, stack):
    for i in range(n):
        stack.callback(print, i)


@with_exit_stack
def locking(lock, stack):
    stack.enter_context(lock)
    # whatever

with_exit_stack的定义如下：

import functools
import contextlib

def with_exit_stack(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        with contextlib.ExitStack() as stack:
            return func(*args, **kwargs, stack=stack)

    return wrapper

Answer 6

我试图使娱乐变得等效（仅作为概念证明进行了测试）

这是：

import os
import inspect


class defer:
    """
    Proof of concept for a python equivalent of golang's defer statement

    Note that the callback order is probably not guaranteed

    """
    def __init__(self, callback, *args, **kwargs):
        self.callback = callback
        self.args = args
        self.kwargs = kwargs

        # Add a reference to self in the caller variables so our __del__
        # method will be called when the function goes out of scope
        caller = inspect.currentframe().f_back
        caller.f_locals[b'_' + os.urandom(48)] = self

    def __del__(self):
        self.callback(*self.args, **self.kwargs)

用法示例：

def main():
    first()
    second()

def first():
    print('- first')
    defer(lambda: print('   - deferred'))
    print('- first exit')

def second():
    print('- second')      

if __name__ == '__main__':
    main()

Python相当于golang的defer语句

6 个答案:

Output