在 python 中捕获性能的异常

Question

我知道 python 中的异常在try时很快，但在捕获时可能很昂贵。

这是否意味着：

try:
   some code
except MyException:
   pass

比这快吗？

try:
   some code
except MyException as e:
   pass

Answer 1

除了 Francesco 的回答之外，似乎捕获的（相对）昂贵的部分之一是异常匹配：

>>> timeit.timeit('try:\n    raise KeyError\nexcept KeyError:\n    pass', number=1000000 )
1.1587663322268327
>>> timeit.timeit('try:\n    raise KeyError\nexcept:\n    pass', number=1000000 )
0.9180641582179874

查看（CPython 2）反汇编：

>>> def f():
...     try:
...         raise KeyError
...     except KeyError:
...         pass
... 
>>> def g():
...     try:
...         raise KeyError
...     except:
...         pass
... 
>>> dis.dis(f)
  2           0 SETUP_EXCEPT            10 (to 13)

  3           3 LOAD_GLOBAL              0 (KeyError)
              6 RAISE_VARARGS            1
              9 POP_BLOCK           
             10 JUMP_FORWARD            17 (to 30)

  4     >>   13 DUP_TOP             
             14 LOAD_GLOBAL              0 (KeyError)
             17 COMPARE_OP              10 (exception match)
             20 POP_JUMP_IF_FALSE       29
             23 POP_TOP             
             24 POP_TOP             
             25 POP_TOP             

  5          26 JUMP_FORWARD             1 (to 30)
        >>   29 END_FINALLY         
        >>   30 LOAD_CONST               0 (None)
             33 RETURN_VALUE        
>>> dis.dis(g)
  2           0 SETUP_EXCEPT            10 (to 13)

  3           3 LOAD_GLOBAL              0 (KeyError)
              6 RAISE_VARARGS            1
              9 POP_BLOCK           
             10 JUMP_FORWARD             7 (to 20)

  4     >>   13 POP_TOP             
             14 POP_TOP             
             15 POP_TOP             

  5          16 JUMP_FORWARD             1 (to 20)
             19 END_FINALLY         
        >>   20 LOAD_CONST               0 (None)
             23 RETURN_VALUE        

请注意，catch 块无论如何都会加载 Exception 并将其与KeyError匹配。 确实，将except KeyError as ke情况：

>>> def f2():
...     try:
...         raise KeyError
...     except KeyError as ke:
...         pass
... 
>>> dis.dis(f2)
  2           0 SETUP_EXCEPT            10 (to 13)

  3           3 LOAD_GLOBAL              0 (KeyError)
              6 RAISE_VARARGS            1
              9 POP_BLOCK           
             10 JUMP_FORWARD            19 (to 32)

  4     >>   13 DUP_TOP             
             14 LOAD_GLOBAL              0 (KeyError)
             17 COMPARE_OP              10 (exception match)
             20 POP_JUMP_IF_FALSE       31
             23 POP_TOP             
             24 STORE_FAST               0 (ke)
             27 POP_TOP             

  5          28 JUMP_FORWARD             1 (to 32)
        >>   31 END_FINALLY         
        >>   32 LOAD_CONST               0 (None)
             35 RETURN_VALUE    

唯一的区别是单个STORE_FAST来存储异常值（在匹配的情况下）。 同样，有几个异常匹配：

>>> def f():
...     try:
...         raise ValueError
...     except KeyError:
...         pass
...     except IOError:
...         pass
...     except SomeOtherError:
...         pass
...     except:
...         pass
... 
>>> dis.dis(f)
  2           0 SETUP_EXCEPT            10 (to 13)

  3           3 LOAD_GLOBAL              0 (ValueError)
              6 RAISE_VARARGS            1
              9 POP_BLOCK           
             10 JUMP_FORWARD            55 (to 68)

  4     >>   13 DUP_TOP             
             14 LOAD_GLOBAL              1 (KeyError)
             17 COMPARE_OP              10 (exception match)
             20 POP_JUMP_IF_FALSE       29
             23 POP_TOP             
             24 POP_TOP             
             25 POP_TOP             

  5          26 JUMP_FORWARD            39 (to 68)

  6     >>   29 DUP_TOP             
             30 LOAD_GLOBAL              2 (IOError)
             33 COMPARE_OP              10 (exception match)
             36 POP_JUMP_IF_FALSE       45
             39 POP_TOP             
             40 POP_TOP             
             41 POP_TOP             

  7          42 JUMP_FORWARD            23 (to 68)

  8     >>   45 DUP_TOP             
             46 LOAD_GLOBAL              3 (SomeOtherError)
             49 COMPARE_OP              10 (exception match)
             52 POP_JUMP_IF_FALSE       61
             55 POP_TOP             
             56 POP_TOP             
             57 POP_TOP             

  9          58 JUMP_FORWARD             7 (to 68)

 10     >>   61 POP_TOP             
             62 POP_TOP             
             63 POP_TOP             

 11          64 JUMP_FORWARD             1 (to 68)
             67 END_FINALLY         
        >>   68 LOAD_CONST               0 (None)
             71 RETURN_VALUE      

将复制异常并尝试将其与列出的每个异常进行匹配，直到找到匹配为止，这（可能）被暗示为“糟糕的捕获性能”。

Answer 2

我认为两者在速度上是一样的：

>>> timeit.timeit('try:\n    raise KeyError\nexcept KeyError:\n    pass', number=1000000 )
0.7168641227143269
>>> timeit.timeit('try:\n    raise KeyError\nexcept KeyError as e:\n    pass', number=1000000 )
0.7733279216613766

Answer 3

捕获并不昂贵，看起来相对较慢的部分是堆栈跟踪本身的创建以及堆栈的后续展开（如果需要）。

我所知道的所有允许您捕获堆栈跟踪的基于堆栈的语言都需要执行这些操作。

1. 当raise被调用时收集堆栈信息。 请注意，Java 1.7 允许您抑制堆栈收集，它的速度要快得多，但您会丢失很多有用的信息。 语言没有明智的方法来知道谁将捕获它，因此忽略异常无济于事，因为无论如何它都必须执行大部分工作。
2. 如果我们引发异常，则展开堆栈，即释放所有内存并展开返回，直到我们遇到有效的捕获为止。

与上述两种操作相比，catch 是微不足道的。 下面是一些代码来证明随着堆栈深度的增加性能会下降。

#!/usr/bin/env python
import os
import re
import time
import pytest

max_depth = 10
time_start = [0] * (max_depth + 1)
time_stop  = [0] * (max_depth + 1)
time_total = [0] * (max_depth + 1)
depth = []
for x in range(0, max_depth):
  depth.append(x)

@pytest.mark.parametrize('i', depth)
def test_stack(benchmark, i):
  benchmark.pedantic(catcher2, args=(i,i), rounds=10, iterations=1000)

#@pytest.mark.parametrize('d', depth)
#def test_recursion(benchmark, d):
#  benchmark.pedantic(catcher, args=(d,), rounds=50, iterations=50) 

def catcher(i, depth):
  try:
    ping(i, depth)
  except Exception:
    time_total[depth] += time.clock() - time_start[depth]

def recurse(i, depth):
  if(d > 0):
    recurse(--i, depth)
  thrower(depth)

def catcher2(i, depth):
  global time_total
  global time_start
  try:
    ping(i, depth)
  except Exception:
    time_total[depth] += time.clock() - time_start[depth]

def thrower(depth):
  global time_start
  time_start[depth] = time.clock()
  raise Exception('wtf')

def ping(i, depth):
  if(i < 1): thrower(i, depth)
  return pong(i, depth)

def pong(i, depth):
  if(i < 0): thrower(i,depth)
  return ping(i - 4, depth)

if __name__ == "__main__":
  rounds     = 200000
  class_time  = 0
  class_start = time.clock()
  for round in range(0, rounds):
    ex = Exception()
  class_time = time.clock() - class_start
  print("%d ex = Exception()'s %f" % (rounds, class_time))

  for depth in range(0, max_depth):
    #print("Depth %d" % depth)
    for round in range(0, rounds):
      catcher(depth, depth)

  for rep in range(0, max_depth):
    print("depth=%d time=%f" % (rep, time_total[rep]/1000000))

输出是，时间（时间是相对的）调用Exception()

200000 ex = Exception()'s 0.040469

depth=0 time=0.103843
depth=1 time=0.246050
depth=2 time=0.401459
depth=3 time=0.565742
depth=4 time=0.736362
depth=5 time=0.921993
depth=6 time=1.102257
depth=7 time=1.278089
depth=8 time=1.463500
depth=9 time=1.657082

比我更py.test Python 的人可能能够让py.test在最后打印时间。

请注意，几周前有一个与此非常相似的关于 Java 的问题。 无论使用何种语言，这都是一个非常有用的线程......

抛出异常的哪一部分是昂贵的？

Answer 4

Python 程序由代码块构成。 块是作为一个单元执行的一段 Python 程序文本。 在 Python 中，核心块表示为 struct basicblock：

cpython/Python/compile.c

typedef struct basicblock_ {
    /* Each basicblock in a compilation unit is linked via b_list in the
       reverse order that the block are allocated.  b_list points to the next
       block, not to be confused with b_next, which is next by control flow. */
    struct basicblock_ *b_list;
    /* number of instructions used */
    int b_iused;
    /* length of instruction array (b_instr) */
    int b_ialloc;
    /* pointer to an array of instructions, initially NULL */
    struct instr *b_instr;
    /* If b_next is non-NULL, it is a pointer to the next
       block reached by normal control flow. */
    struct basicblock_ *b_next;
    /* b_seen is used to perform a DFS of basicblocks. */
    unsigned b_seen : 1;
    /* b_return is true if a RETURN_VALUE opcode is inserted. */
    unsigned b_return : 1;
    /* depth of stack upon entry of block, computed by stackdepth() */
    int b_startdepth;
    /* instruction offset for block, computed by assemble_jump_offsets() */
    int b_offset;
} basicblock;

循环、try/except 和 try/finally 语句处理了不同的事情。 对于这 3 个语句，使用了框架块：

cpython/Python/compile.c

enum fblocktype { LOOP, EXCEPT, FINALLY_TRY, FINALLY_END };

struct fblockinfo {
    enum fblocktype fb_type;
    basicblock *fb_block;
};

代码块在执行帧中执行。

cpython/包含/frameobject.h

typedef struct _frame {
    PyObject_VAR_HEAD
    struct _frame *f_back;      /* previous frame, or NULL */
    PyCodeObject *f_code;       /* code segment */
    PyObject *f_builtins;       /* builtin symbol table (PyDictObject) */
    PyObject *f_globals;        /* global symbol table (PyDictObject) */
    PyObject *f_locals;         /* local symbol table (any mapping) */
    PyObject **f_valuestack;    /* points after the last local */
    /* Next free slot in f_valuestack.  Frame creation sets to f_valuestack.
       Frame evaluation usually NULLs it, but a frame that yields sets it
       to the current stack top. */
    PyObject **f_stacktop;
    PyObject *f_trace;          /* Trace function */

    /* In a generator, we need to be able to swap between the exception
       state inside the generator and the exception state of the calling
       frame (which shouldn't be impacted when the generator "yields"
       from an except handler).
       These three fields exist exactly for that, and are unused for
       non-generator frames. See the save_exc_state and swap_exc_state
       functions in ceval.c for details of their use. */
    PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
    /* Borrowed reference to a generator, or NULL */
    PyObject *f_gen;

    int f_lasti;                /* Last instruction if called */
    /* Call PyFrame_GetLineNumber() instead of reading this field
       directly.  As of 2.3 f_lineno is only valid when tracing is
       active (i.e. when f_trace is set).  At other times we use
       PyCode_Addr2Line to calculate the line from the current
       bytecode index. */
    int f_lineno;               /* Current line number */
    int f_iblock;               /* index in f_blockstack */
    char f_executing;           /* whether the frame is still executing */
    PyTryBlock f_blockstack[CO_MAXBLOCKS]; /* for try and loop blocks */
    PyObject *f_localsplus[1];  /* locals+stack, dynamically sized */
} PyFrameObject;

帧包含一些管理信息（用于调试）并确定代码块执行完成后继续执行的位置和方式。 当您使用 'as' 语句（在 'import something as' 或 'except Exception as' 语句中）时，您只需执行名称绑定操作。 即 Python 只需在框架对象的 *f_locals 符号表中添加对对象的引用。 因此不会在运行时没有开销。

但是在解析时你会有一些开销。

cpython/Modules/parsermodule.c

static int
validate_except_clause(node *tree)
{
    int nch = NCH(tree);
    int res = (validate_ntype(tree, except_clause)
               && ((nch == 1) || (nch == 2) || (nch == 4))
               && validate_name(CHILD(tree, 0), "except"));

    if (res && (nch > 1))
        res = validate_test(CHILD(tree, 1));
    if (res && (nch == 4))
        res = (validate_name(CHILD(tree, 2), "as")
               && validate_ntype(CHILD(tree, 3), NAME));

    return (res);
}

但是，在我看来，这可以忽略