The short answer, or TL;DR
Basically, eval
is used to evaluate a single dynamically generated Python expression, and exec
is used to execute dynamically generated Python code only for its side effects.
eval
and exec
have these two differences:
eval
accepts only a single expression, exec
can take a code block that has Python statements: loops, try: except:
, class
and function/method def
initions and so on.
An expression in Python is whatever you can have as the value in a variable assignment:
a_variable = (anything you can put within these parentheses is an expression)
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).
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.
In Python 3, exec
is a function; its use has no effect on the compiled bytecode of the function where it is used.
Thus basically:
>>> 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
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
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
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.
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 compile
d it into bytecode before instead of passing it as a string:
>>> eval(compile('if 1: print("Hello")', '<string>', 'exec'))
Hello
>>>
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
.
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
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
>>>
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
-<cod