Because x=1
is a statement, not an expression. Use exec
to run statements.
>>> exec('x=1')
>>> x
1
By the way, there are many ways to avoid using exec
/eval
if all you need is a dynamic name to assign, e.g. you could use a dictionary, the setattr
function, or the :locals()
dictionary
>>> locals()['y'] = 1
>>> y
1
Update: Although the code above works in the REPL, it won’t work inside a function. See Modifying locals in Python for some alternatives if exec
is out of question.