Added section on order of evaluation.

pep-0498.txt

@@ -602,10 +602,24 @@ having 2 expressions::
 
   f'{x:.{width}}'
 
-Expressions with side effects
+Evaluation order of expressions
------------------------------
+-------------------------------
 
-xxx
+The expressions in an f-string are evaluated in left-to-right
+order. This is detectable only if the expressions have side effects::
+
+  >>> def fn(l, incr):
+  ...    result = l[0]
+  ...    l[0] += incr
+  ...    return result
+  ...
+  >>> lst = [0]
+  >>> f'{fn(lst,2)} {fn(lst,3)}'
+  '0 2'
+  >>> f'{fn(lst,2)} {fn(lst,3)}'
+  '5 7'
+  >>> lst
+  [10]
 
 Expressions used multiple times
 -------------------------------