CODEWITHSUNDEEP
pythonbuilt-in
F.S.
F.S.

Reputation: 1261

When to use "eval" mode rather than "exec" with Python compile()

Python's built-in compile function has eval, exec and single modes. single mode seems a bit useless (ref). How about the eval and exec?

But an expression is a statement. My question is: why not just use exec all the time?

PS: I've read some related questions like Python built-in function "compile". What is it used for?. They do a good job explaining the difference between the modes, but doesn't directly answer my question.

Upvotes: 2

Views: 1067

Answers (3)

MisterMiyagi
MisterMiyagi

Reputation: 52089

Just like the eval builtin, the compile eval mode creates a code object that evaluates an expression and returns its result. In contrast, exec mode does not return a result.

>>> exec_code = compile("1 + 2", "<stack overflow>", "exec")
>>> eval(exec_code)  # no result for "exec" mode code
>>> eval_code = compile("1 + 2", "<stack overflow>", "eval")
>>> eval(eval_code)  # some result for "eval" mode code
3

But an expression is a statement.

This is not true. An expression evaluates to a value, a statement does not.

While expression statements mean an expression can be used "as a" statement, that does not make the two equivalent. An expression statement contains an expression; when run, the statement evaluates the expression but discards the result.

Upvotes: 4

oldpride
oldpride

Reputation: 975

updated and extra examples.

The accepted answer's examples do not working in python 3.10 any more.

>> exec_code = eval(compile("1 + 2", "<stack overflow>", "exec"))
>>> eval(exec_code)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: eval() arg 1 must be a string, bytes or code object

Updated examples for python 3.10

>> exec_code = compile("1 + 2", "<stack overflow>", "exec")
>>> exec(exec_code)
>>> eval_code = compile("1 + 2", "<stack overflow>", "eval")
>>> eval(eval_code)  # some result for "eval" mode code
3

Example of a block of code

>>> a='''
... b=1+4
... print(b)
... '''
>>> exec_code = compile(a, "<stack overflow>", "exec")
>>> exec(exec_code)
5

Upvotes: 0

chepner
chepner

Reputation: 532093

An expression is not a statement, but there is a kind of statement (the expression statement) that consists of a single expression. This nifty bit of indirection allows the parser to think a Python module as a sequence of statements, rather than a sequence of statements and/or expressions.

An expression has a value; an expression statement does not: it simply evaluates the expression (primarily for its side effects) and discards the value.

eval returns the value of the evaluated expression.

exec will execute an expression as an expression statement, but discard the value of the expression. (exec itself is a function which is primarily called in expression statements, rather than as part of a larger expression.)

Upvotes: 4

Related Questions