eval 返回给定表达式的值，而 exec 忽略其代码的返回值，并始终返回 None(在 Python 2 中它是一个语句，不能用作表达式，所以它真的不返回任何东西).

eval returns the value of the given expression, whereas exec ignores the return value from its code, and always returns None (in Python 2 it is a statement and cannot be used as an expression, so it really does not return anything).

在 1.0 - 2.7 版本中，exec 是一个语句，因为 CPython 需要为使用 exec 的函数生成不同类型的代码对象，因为它在功能.

In versions 1.0 - 2.7, exec was a statement, because CPython needed to produce a different kind of code object for functions that used exec for its side effects inside the function.

在 Python 3 中，exec 是一个函数；它的使用对使用它的函数的编译字节码没有影响.

In Python 3, exec is a function; its use has no effect on the compiled bytecode of the function where it is used.

因此基本上:

>>> a = 5
>>> eval('37 + a')   # it is an expression
42
>>> exec('37 + a')   # it is an expression statement; value is ignored (None is returned)
>>> exec('a = 47')   # modify a global variable as a side effect
>>> a
47
>>> eval('a = 47')  # you cannot evaluate a statement
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    a = 47
      ^
SyntaxError: invalid syntax

<小时>

'exec' 模式下的 compile 将任意数量的语句编译成隐式总是返回 None 的字节码，而在 'eval' 模式将单个表达式编译成字节码，返回该表达式的值.

The compile in 'exec' mode compiles any number of statements into a bytecode that implicitly always returns None, whereas in 'eval' mode it compiles a single expression into bytecode that returns the value of that expression.

>>> eval(compile('42', '<string>', 'exec'))  # code returns None
>>> eval(compile('42', '<string>', 'eval'))  # code returns 42
42
>>> exec(compile('42', '<string>', 'eval'))  # code returns 42,
>>>                                          # but ignored by exec

在 'eval' 模式下(如果传入字符串，则使用 eval 函数)，compile 会引发异常如果源代码包含语句或超出单个表达式的任何其他内容:

In the 'eval' mode (and thus with the eval function if a string is passed in), the compile raises an exception if the source code contains statements or anything else beyond a single expression:

>>> compile('for i in range(3): print(i)', '<string>', 'eval')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    for i in range(3): print(i)
      ^
SyntaxError: invalid syntax

<小时>

实际上，eval 只接受一个表达式" 语句仅适用于将字符串(包含 Python 源代码)传递给 eval.然后使用 compile(source, '<string> 在内部将其编译为字节码;', 'eval') 这才是真正的区别所在.

Actually the statement "eval accepts only a single expression" applies only when a string (which contains Python source code) is passed to eval. Then it is internally compiled to bytecode using compile(source, '<string>', 'eval') This is where the difference really comes from.

如果将 code 对象(包含 Python bytecode)传递给 exec 或 eval，它们的行为相同，除了 exec 忽略返回值，仍然总是返回 None 的事实.所以可以使用 eval 来执行有语句的东西，如果你只是 compiled 之前把它变成字节码，而不是把它作为字符串传递:

If a code object (which contains Python bytecode) is passed to exec or eval, they behave identically, excepting for the fact that exec ignores the return value, still returning None always. So it is possible use eval to execute something that has statements, if you just compiled it into bytecode before instead of passing it as a string:

>>> eval(compile('if 1: print("Hello")', '<string>', 'exec'))
Hello
>>>

即使编译后的代码包含语句，也可以正常工作.它仍然返回 None，因为那是 compile 返回的代码对象的返回值.

works without problems, even though the compiled code contains statements. It still returns None, because that is the return value of the code object returned from compile.

在 'eval' 模式下(如果传入字符串，则使用 eval 函数)，compile 会引发异常如果源代码包含语句或超出单个表达式的任何其他内容:

In the 'eval' mode (and thus with the eval function if a string is passed in), the compile raises an exception if the source code contains statements or anything else beyond a single expression:

>>> compile('for i in range(3): print(i)', '<string>'. 'eval')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    for i in range(3): print(i)
      ^
SyntaxError: invalid syntax

更长的答案，也就是血淋淋的细节

exec 和 eval

exec 函数(其中Python 2 中的语句) 用于执行动态创建的语句或程序:

The longer answer, a.k.a the gory details

exec and eval

The exec function (which was a statement in Python 2) is used for executing a dynamically created statement or program:

>>> program = '''
for i in range(3):
    print("Python is cool")
'''
>>> exec(program)
Python is cool
Python is cool
Python is cool
>>> 

eval 函数执行单个表达式 相同，and 返回值表达式:

The eval function does the same for a single expression, and returns the value of the expression:

>>> a = 2
>>> my_calculation = '42 * a'
>>> result = eval(my_calculation)
>>> result
84

exec 和 eval 都接受作为 str、unicode 或包含源代码的 bytes 对象，或作为包含 Python 字节码的 code 对象.

exec and eval both accept the program/expression to be run either as a str, unicode or bytes object containing source code, or as a code object which contains Python bytecode.

如果将包含源代码的 str/unicode/bytes 传递给 exec，则其行为等同于:

If a str/unicode/bytes containing source code was passed to exec, it behaves equivalently to:

exec(compile(source, '<string>', 'exec'))

和 eval 的行为类似地等同于:

and eval similarly behaves equivalent to:

eval(compile(source, '<string>', 'eval'))

<小时>

由于所有表达式都可以用作 Python 中的语句(这些在 Python 中称为 Expr 节点抽象语法；反之则不然)，如果不需要返回值，可以随时使用exec.也就是说，你可以使用 eval('my_func(42)') 或 exec('my_func(42)')，区别在于 eval 返回 my_func 返回的值，exec 将其丢弃:

Since all expressions can be used as statements in Python (these are called the Expr nodes in the Python abstract grammar; the opposite is not true), you can always use exec if you do not need the return value. That is to say, you can use either eval('my_func(42)') or exec('my_func(42)'), the difference being that eval returns the value returned by my_func, and exec discards it:

>>> def my_func(arg):
...     print("Called with %d" % arg)
...     return arg * 2
... 
>>> exec('my_func(42)')
Called with 42
>>> eval('my_func(42)')
Called with 42
84
>>> 

在这 2 个中，只有 exec 接受包含语句的源代码，例如 def、for、while、import 或 class、赋值语句(又名 a = 42)或整个程序:

Of the 2, only exec accepts source code that contains statements, like def, for, while, import, or class, the assignment statement (a.k.a a = 42), or entire programs:

>>> exec('for i in range(3): print(i)')
0
1
2
>>> eval('for i in range(3): print(i)')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    for i in range(3): print(i)
      ^
SyntaxError: invalid syntax

<小时>

exec 和 eval 都接受 2 个额外的位置参数 - globals 和 locals - 它们是全局和代码看到的局部变量范围.这些默认为调用 exec 或 eval 的范围内的 globals() 和 locals()，但任何字典可用于 globals 和任何 mapping 用于 locals(当然包括 dict).这些不仅可以用来限制/修改代码看到的变量，还经常用于捕获 executed 代码创建的变量:

Both exec and eval accept 2 additional positional arguments - globals and locals - which are the global and local variable scopes that the code sees. These default to the globals() and locals() within the scope that called exec or eval, but any dictionary can be used for globals and any mapping for locals (including dict of course). These can be used not only to restrict/modify the variables that the code sees, but are often also used for capturing the variables that the executed code creates:

>>> g = dict()
>>> l = dict()
>>> exec('global a; a, b = 123, 42', g, l)
>>> g['a']
123
>>> l
{'b': 42}

(如果显示整个g的值，会长很多，因为exec和eval加了内置模块作为 __builtins__ 自动添加到全局变量(如果它丢失).

(If you display the value of the entire g, it would be much longer, because exec and eval add the built-ins module as __builtins__ to the globals automatically if it is missing).

在Python 2中，exec语句的官方语法实际上是exec code in globals, locals，如

In Python 2, the official syntax for the exec statement is actually exec code in globals, locals, as in

>>> exec 'global a; a, b = 123, 42' in g, l

然而，替代语法 exec(code, globals, locals) 也一直被接受(见下文).

However the alternate syntax exec(code, globals, locals) has always been accepted too (see below).

compile(source, filename, mode, flags=0,dont_inherit=False, optimize=-1) 内置可用于加速使用 exec 或 eval 重复调用相同代码通过预先将源代码编译成 code 对象.mode 参数控制 compile 函数接受的代码片段类型及其生成的字节码类型.选择是 'eval'、'exec' 和 'single':

The compile(source, filename, mode, flags=0, dont_inherit=False, optimize=-1) built-in can be used to speed up repeated invocations of the same code with exec or eval by compiling the source into a code object beforehand. The mode parameter controls the kind of code fragment the compile function accepts and the kind of bytecode it produces. The choices are 'eval', 'exec' and 'single':

• 'eval' 模式需要一个表达式，并且会生成字节码，运行时会返回那个表达式的值:

• 'eval' mode expects a single expression, and will produce bytecode that when run will return the value of that expression:

>>> dis.dis(compile('a + b', '<string>', 'eval'))
  1           0 LOAD_NAME                0 (a)
              3 LOAD_NAME                1 (b)
              6 BINARY_ADD
              7 RETURN_VALUE

'exec' 接受从单个表达式到整个代码模块的任何类型的 python 构造，并像执行模块顶级语句一样执行它们.代码对象返回None:

'exec' accepts any kinds of python constructs from single expressions to whole modules of code, and executes them as if they were module top-level statements. The code object returns None:

>>> dis.dis(compile('a + b', '<string>', 'exec'))
  1           0 LOAD_NAME                0 (a)
              3 LOAD_NAME                1 (b)
              6 BINARY_ADD
              7 POP_TOP                             <- discard result
              8 LOAD_CONST               0 (None)   <- load None on stack
             11 RETURN_VALUE                        <- return top of stack

'single' 是 'exec' 的有限形式，它接受包含 single 语句(或多个由 ; 分隔的语句)如果最后一个语句是表达式语句，则生成的字节码也 将该表达式的值的 repr 打印到标准输出(！).

'single' is a limited form of 'exec' which accepts a source code containing a single statement (or multiple statements separated by ;) if the last statement is an expression statement, the resulting bytecode also prints the repr of the value of that expression to the standard output(!).

if-elif-else 链，带有 else 和 try<的循环/code> 及其 except、else 和 finally 块被视为单个语句.

>>> exec(compile('a = 5
a = 6', '<string>', 'single'))
>>> a
5

>>> exec(compile('a = 5
a = 6', '<string>', 'single'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    a = 5
        ^
SyntaxError: multiple statements found while compiling a single statement

>>> eval('for i in range(3): print("Python is cool")')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    for i in range(3): print("Python is cool")
      ^
SyntaxError: invalid syntax

>>> code = compile('for i in range(3): print("Python is cool")',
                   'foo.py', 'exec')
>>> eval(code)
Python is cool
Python is cool
Python is cool

>>> exec("print(a)", globals(), {'a': 42})
42

Python 2.7.11+ (default, Apr 17 2016, 14:00:29) 
[GCC 5.3.1 20160413] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> a = exec('print(42)')
  File "<stdin>", line 1
    a = exec('print(42)')
           ^
SyntaxError: invalid syntax

>>> call_later(exec, 'print(42)', delay=1000)
  File "<stdin>", line 1
    call_later(exec, 'print(42)', delay=1000)
                  ^
SyntaxError: invalid syntax

>>> [exec(i) for i in ['print(42)', 'print(foo)']
  File "<stdin>", line 1
    [exec(i) for i in ['print(42)', 'print(foo)']
        ^
SyntaxError: invalid syntax