Python: Way to speed up a repeatedly executed eval statement?

Question

In my code, I'm using eval to evaluate a string expression given by the user. Is there a way to compile or otherwise speed up this statement?

import math import random  result_count = 100000 expression = "math.sin(v['x']) * v['y']"  variable = dict() variable['x'] = [random.random() for _ in xrange(result_count)] variable['y'] = [random.random() for _ in xrange(result_count)]  # optimize anything below this line  result = [0] * result_count  print 'Evaluating %d instances of the given expression:' % result_count print expression  v = dict() for index in xrange(result_count):     for name in variable.keys():         v[name] = variable[name][index]     result[index] = eval(expression) # <-- option ONE     #result[index] = math.sin(v['x']) * v['y'] # <-- option TWO 

For a quick comparison option ONE takes 2.019 seconds on my machine, while option TWO takes only 0.218 seconds. Surely Python has a way of doing this without hard-coding the expression.

Answer 1

You can also trick python:

expression = "math.sin(v['x']) * v['y']" exp_as_func = eval('lambda: ' + expression) 

And then use it like so:

exp_as_func() 

Speed test:

In [17]: %timeit eval(expression) 10000 loops, best of 3: 25.8 us per loop  In [18]: %timeit exp_as_func() 1000000 loops, best of 3: 541 ns per loop 

---

As a side note, if v is not a global, you can create the lambda like this:

exp_as_func = eval('lambda v: ' + expression) 

and call it:

exp_as_func(my_v) 

Answer 2

You can avoid the overhead by compiling the expression in advance using compiler.compile() for Python 2 or compile() for Python 3 :

In [1]: import math, compiler  In [2]: v = {'x': 2, 'y': 4}  In [3]: expression = "math.sin(v['x']) * v['y']"  In [4]: %timeit eval(expression) 10000 loops, best of 3: 19.5 us per loop  In [5]: compiled = compiler.compile(expression, '<string>', 'eval')  In [6]: %timeit eval(compiled) 1000000 loops, best of 3: 823 ns per loop 

Just make sure you do the compiling only once (outside of the loop). As mentioned in comments, when using eval on user submitted strings make sure you are very careful about what you accept.