You're probably looking for eval(): https://docs.python.org/3.5/library/functions.html#eval

p1 = 100
print(eval('p1'))   # prints 100

If you want to evaluate expressions you can also use exec()

p1 = 100
t = 'p1 += 99'
exec(t)
print(p1)           # prints 199

But as other already pointed out, you should avoid it if possible. timgeb provided a nice alternative with dictionaries for example.

If you want to turn a string into a global variable you could also do:

globals()['y'] = 5
print(y)

Note: when you run your code in a global scope (outside a function), you can also use locals()['y'] = 5 - but this will not work in a non-global scope!

This works in many popular languages like JavaScript and Ruby, why not in Python?

>>> ['a', 'b', 'c'].join('')
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'join'

Strange enough, in Python the join method is on the str class:

# this is the Python way
"".join(['a','b','c','d'])

Why join is not a method in the list object like in JavaScript or other popular script languages? It is one example of how the Python community thinks. Since join is returning a string, it should be placed in the string class, not on the list class, so the str.join(list) method means: join the list into a new string using str as a separator (in this case str is an empty string).

Somehow I got to love this way of thinking after a while. I can complain about a lot of things in Python design, but not about its coherence.