В чем разница между eval, exec и compile?
Я изучал динамическую оценку кода Python и наткнулся на функции eval()
and compile()
и exec
оператор.
Может кто-нибудь, пожалуйста, объяснить разницу между eval
и exec
, и как сочетаются разные режимы compile()
?
Переведено автоматически
Ответ 1
В основном, eval
используется для оценки одного динамически генерируемого выражения Python и exec
используется для выполнения динамически генерируемого кода Python только для его побочных эффектов.
eval
и exec
есть ли эти два отличия:
eval
принимает только единственное выражение, exec
может принимать блок кода, содержащий инструкции Python: циклы, try: except:
, class
и инициалы функции / метода def
и так далее.
Выражение в Python - это все, что вы можете использовать в качестве значения при присваивании переменной:
a_variable = (anything you can put within these parentheses is an expression)
eval
возвращает значение данного выражения, тогда как exec
игнорирует возвращаемое значение из своего кода и всегда возвращает None
(в Python 2 это оператор, и его нельзя использовать как выражение, поэтому он действительно ничего не возвращает).
В версиях 1.0 - 2.7, exec
был оператором, потому что CPython требовалось создать другой тип объекта кода для функций, которые использовались exec
для его побочных эффектов внутри функции.
В Python 3 exec
- это функция; ее использование не влияет на скомпилированный байт-код функции, в которой она используется.
Таким образом, в основном:
>>> 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
Режим compile
in 'exec'
компилирует любое количество операторов в байт-код, который неявно всегда возвращает None
, тогда как в 'eval'
режиме он компилирует единственное выражение в байт-код, который возвращает значение этого выражения.
>>> 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
В 'eval'
режиме (и, следовательно, с eval
функцией, если передается строка), compile
вызывает исключение, если исходный код содержит инструкции или что-либо еще, кроме одного выражения:
>>> 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
На самом деле утверждение "eval принимает только одно выражение" применяется только тогда, когда в передается строка (которая содержит исходный код на Pythoneval
). Затем он внутренне компилируется в байт-код с использованием compile(source, '<string>', 'eval')
Вот откуда на самом деле берется разница.
Если code
объект (который содержит байт-код Python) передается в exec
or eval
, они ведут себя идентично, за исключением того факта, что exec
игнорирует возвращаемое значение, продолжая возвращать None
всегда. Таким образом, можно использовать eval
для выполнения чего-либо, содержащего инструкции, если вы просто compile
передали это в байт-код, а не передавали как строку:
>>> eval(compile('if 1: print("Hello")', '<string>', 'exec'))
Hello
>>>
работает без проблем, даже если скомпилированный код содержит инструкции. Он все равно возвращает None
, потому что это возвращаемое значение объекта кода, возвращаемого из compile
.
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
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
>>>
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
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.
If a str
/unicode
/bytes
containing source code was passed to exec
, it behaves equivalently to:
exec(compile(source, '<string>', 'exec'))
and eval
similarly behaves equivalent to:
eval(compile(source, '<string>', 'eval'))
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
>>>
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
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 exec
uted code creates:
>>> g = dict()
>>> l = dict()
>>> exec('global a; a, b = 123, 42', g, l)
>>> g['a']
123
>>> l
{'b': 42}
(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).
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
However the alternate syntax exec(code, globals, locals)
has always been accepted too (see below).
compile
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'
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'
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'
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(!).
An if
-elif
-else
chain, a loop with else
, and try
with its except
, else
and finally
blocks is considered a single statement.
A source fragment containing 2 top-level statements is an error for the 'single'
, except in Python 2 there is a bug that sometimes allows multiple toplevel statements in the code; only the first is compiled; the rest are ignored:
In Python 2.7.8:
>>> exec(compile('a = 5\na = 6', '<string>', 'single'))
>>> a
5
And in Python 3.4.2:
>>> exec(compile('a = 5\na = 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
This is very useful for making interactive Python shells. However, the value of the expression is not returned, even if you eval
the resulting code.
Thus greatest distinction of exec
and eval
actually comes from the compile
function and its modes.
In addition to compiling source code to bytecode, compile
supports compiling abstract syntax trees (parse trees of Python code) into code
objects; and source code into abstract syntax trees (the ast.parse
is written in Python and just calls compile(source, filename, mode, PyCF_ONLY_AST)
); these are used for example for modifying source code on the fly, and also for dynamic code creation, as it is often easier to handle the code as a tree of nodes instead of lines of text in complex cases.
While eval
only allows you to evaluate a string that contains a single expression, you can eval
a whole statement, or even a whole module that has been compile
d into bytecode; that is, with Python 2, print
is a statement, and cannot be eval
led directly:
>>> 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
compile
it with 'exec'
mode into a code
object and you can eval
it; the eval
function will return None
.
>>> 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
If one looks into eval
and exec
source code in CPython 3, this is very evident; they both call PyEval_EvalCode
with same arguments, the only difference being that exec
explicitly returns None
.
exec
between Python 2 and Python 3One of the major differences in Python 2 is that exec
is a statement and eval
is a built-in function (both are built-in functions in Python 3).
It is a well-known fact that the official syntax of exec
in Python 2 is exec code [in globals[, locals]]
.
Unlike majority of the Python 2-to-3 porting guides seem to suggest, the exec
statement in CPython 2 can be also used with syntax that looks exactly like the exec
function invocation in Python 3. The reason is that Python 0.9.9 had the exec(code, globals, locals)
built-in function! And that built-in function was replaced with exec
statement somewhere before Python 1.0 release.
Since it was desirable to not break backwards compatibility with Python 0.9.9, Guido van Rossum added a compatibility hack in 1993: if the code
was a tuple of length 2 or 3, and globals
and locals
were not passed into the exec
statement otherwise, the code
would be interpreted as if the 2nd and 3rd element of the tuple were the globals
and locals
respectively. The compatibility hack was not mentioned even in Python 1.4 documentation (the earliest available version online); and thus was not known to many writers of the porting guides and tools, until it was documented again in November 2012:
The first expression may also be a tuple of length 2 or 3. In this case, the optional parts must be omitted. The form
exec(expr, globals)
is equivalent toexec expr in globals
, while the formexec(expr, globals, locals)
is equivalent toexec expr in globals, locals
. The tuple form ofexec
provides compatibility with Python 3, whereexec
is a function rather than a statement.
Yes, in CPython 2.7 that it is handily referred to as being a forward-compatibility option (why confuse people over that there is a backward compatibility option at all),
when it actually had been there for backward-compatibility for two decades.
Thus while exec
is a statement in Python 1 and Python 2, and a built-in function in Python 3 and Python 0.9.9,
>>> exec("print(a)", globals(), {'a': 42})
42
has had identical behaviour in possibly every widely released Python version ever; and works in Jython 2.5.2, PyPy 2.3.1 (Python 2.7.6) and IronPython 2.6.1 too (kudos to them following the undocumented behaviour of CPython closely).
What you cannot do in Pythons 1.0 - 2.7 with its compatibility hack, is to store the return value of exec
into a variable:
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
(which wouldn't be useful in Python 3 either, as exec
always returns None
), or pass a reference to exec
:
>>> call_later(exec, 'print(42)', delay=1000)
File "<stdin>", line 1
call_later(exec, 'print(42)', delay=1000)
^
SyntaxError: invalid syntax
Which a pattern that someone might actually have used, though unlikely;
Or use it in a list comprehension:
>>> [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
which is abuse of list comprehensions (use a for
loop instead!).
Ответ 2
exec
is not an expression: a statement in Python 2.x, and a function in Python 3.x. It compiles and immediately evaluates a statement or set of statement contained in a string. Example:
exec('print(5)') # prints 5.
# exec 'print 5' if you use Python 2.x, nor the exec neither the print is a function there
exec('print(5)\nprint(6)') # prints 5{newline}6.
exec('if True: print(6)') # prints 6.
exec('5') # does nothing and returns nothing.
eval
is a built-in function (not a statement), which evaluates an expression and returns the value that expression produces. Example:
x = eval('5') # x <- 5
x = eval('%d + 6' % x) # x <- 11
x = eval('abs(%d)' % -100) # x <- 100
x = eval('x = 5') # INVALID; assignment is not an expression.
x = eval('if 1: x = 4') # INVALID; if is a statement, not an expression.
compile
is a lower level version of exec
and eval
. It does not execute or evaluate your statements or expressions, but returns a code object that can do it. The modes are as follows:
compile(string, '', 'eval')
returns the code object that would have been executed had you done eval(string)
. Note that you cannot use statements in this mode; only a (single) expression is valid.
compile(string, '', 'exec')
returns the code object that would have been executed had you done exec(string)
. You can use any number of statements here.
compile(string, '', 'single')
is like the exec
mode but expects exactly one expression/statement, eg compile('a=1 if 1 else 3', 'myf', mode='single')
Ответ 3
exec is for statement and does not return anything.
eval is for expression and returns value of expression.
expression means "something" while statement means "do something".