Why must “exec” (and not “eval”) be used for Python import statements?

问题

I'm trying to run a snippet of Python from within Java, using Jython. If I use an exec statement to import, everything works.

PythonInterpreter pi = new PythonInterpreter();
pi.exec("import re");
PythonObject o = pi.eval("re.match('abc', 'abc123')"); // returns a MatchObject
o = pi.eval("re.match('abc', 'def123')"); // returns Py.None

If, however, I try to combine the two lines, all hell breaks loose. This:

PythonInterpreter pi = new PythonInterpreter();
pi.eval("import re"); // exception!
PythonObject o = pi.eval("re.match('abc', 'abc123')"); // never gets here
o = pi.eval("re.match('abc', 'def123')"); // ....

...throws an exception "no viable alternative at input 'import'", ('<string>',1,0,'import re\n').

This matters, because ideally I'd like to be able to eval a whole script as a single string, without having to break the imports out into a separate part. Am I doing something wrong? Is there another way to tell Jython "take this whole blob of script, including imports, and run it, then give me back a result"? This needs to be at runtime -- pre-compiling the Python into .class files is not an option.

回答1:

The problem is that eval evaluates expressions and returns some result, while exec executes statements in some context. import is a statement, while re.match() is an expression.

回答2:

Try this,

eval("__import__('re').match('abc', 'abc123')")

回答3:

Here what do you mean by the result?

I am assuming that it is some calculated value of your script. According to the documentation of 'exec' the code runs in the current scope. Hence you must be able to access the values of any new symbols that you have created inside exec statement.

a = 20
b = 10
exec("c = a + b")
print c

The above code should print 30.

来源：https://stackoverflow.com/questions/11886540/why-must-exec-and-not-eval-be-used-for-python-import-statements

标签

python

eval

jython