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

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.