How to change variable from list to float

[Gary Barker]

I have searched for a solution to this but have not found a suitable 
example.

The attached code generates this error: Traceback (most recent call last):
   File "calcsignal.py", line 7, in <module>
     siglevfromexist = 34.8 + existattn
TypeError: unsupported operand type(s) for +: 'float' and 'list'

How do I convert the list variable (i.e. existattn) to a float?

Operating details are:
Python 3.4.2
Debian 3.16.39-1+deb8u2 (2017-03-07) x86_64 GNU/Linux

The following lines are the code in calcsignal.py:
azdegpattrev = -47.40715077970316
azattndic = {-0.9: [-0.55], -0.5: [-0.46], 3.5: [-21.0], 1.4: [-7.48], 
5.5: [-25.0], 0.0: [0.0], 1.9: [-21.0], 13.0: [-38.0], 15.0: [-39.0], 
3.6: [-25.0], 20.0: [-39.0], -1.4: [-7.48], 90.0: [-65.0], -0.4: 
[-0.39], 0.5: [-0.46], 0.1: [-0.04], 1.0: [-1.31], -90.0: [-65.0], 40.0: 
[-42.0], 180.0: [-65.0], 1.5: [-10.0], -1.2: [-3.69], 0.3: [-0.28], 
-0.3: [-0.28], 0.2: [-0.15], -0.1: [-0.04], 1.1: [-2.34], -180.0: 
[-65.0], -0.2: [-0.15], 1.2: [-3.69], -40.0: [-42.0], 0.4: [-0.39], 
-5.5: [-25.0], -1.5: [-10.0], -20.0: [-39.0], 0.9: [-0.55], -3.5: 
[-21.0], -1.9: [-21.0], -15.0: [-39.0], -13.0: [-38.0], 1.3: [-5.39], 
-1.3: [-5.39], -3.6: [-25.0], -1.0: [-1.31], -1.1: [-2.34]}
azlist = [-0.9, -0.5, 3.5, 1.4, 5.5, 0.0, 1.9, 13.0, 15.0, 3.6, 20.0, 
-1.4, 90.0, -0.4, 0.5, 0.1, 1.0, -90.0, 40.0, 180.0, 1.5, -1.2, 0.3, 
-0.3, 0.2, -0.1, 1.1, -180.0, -0.2, 1.2, -40.0, 0.4, -5.5, -1.5, -20.0, 
0.9, -3.5, -1.9, -15.0, -13.0, 1.3, -1.3, -3.6, -1.0, -1.1]
azlist = [float(i) for i in azlist]
closestaz = min(azlist, key=lambda x: abs(x - azdegpattrev))
existattn = azattndic[closestaz]
siglevfromexist = 34.8 + existattn