How can I assign the value of a variable using eval in python?

Because x=1 is a statement, not an expression. Use exec to run statements.

>>> exec('x=1')
>>> x
1

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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.

You can't, since variable assignment is a statement, not an expression, and eval can only eval expressions. Use exec instead.

Better yet, don't use either and tell us what you're really trying to do so that we can come up with a safe and sane solution.