A quick single-file MIT-Licenced library for easily adding evaluatable expressions into python projects. Say you want to allow a user to set an alarm volume, which could depend on the time of day, alarm level, how many previous alarms had gone off, and if there is music playing at the time.
Or if you want to allow simple formulae in a web application, but don't want to
give full eval()
access, or don't want to run in javascript on the client side.
It's deliberately very simple, just a single file you can dump into a project, or import from pypi (pip or easy_install).
Internally, it's using the amazing python ast
module to parse the expression, which
allows very fine control of what is and isn't allowed. It should be completely safe in terms
of what operations can be performed by the expression.
The only issue I know to be aware of is that you can create an expression which
takes a long time to evaluate, or which evaluating requires an awful lot of memory,
which leaves the potential for DOS attacks. There is basic protection against this,
and you can lock it down further if you desire. (see the Operators
section below)
You should be aware of this when deploying in a public setting.
The defaults are pretty locked down and basic, and it's very easy to add whatever extra specific functionality you need (your own functions, variable/name lookup, etc).
Either just copy the simpleeval.py file into your project, or use pip:
pip install simpleeval
(or add simpleeval into your requirements.txt, if that's how your project works...)
To get very simple evaluating:
from simpleeval import simple_eval
simple_eval("21 + 21")
returns 42
.
Expressions can be as complex and convoluted as you want:
simple_eval("21 + 19 / 7 + (8 % 3) ** 9")
returns 535.714285714
.
You can add your own functions in as well.
simple_eval("square(11)", functions={"square": lambda x: x*x})
returns 121
.
For more details of working with functions, read The docs on pypi
all further examples use >>>
to designate python code, as if you are using the python interactive
prompt.
Also note, the **
operator has been locked down by default to have a maximum input value
of 4000000
, which makes it somewhat harder to make expressions which go on for ever. You
can change this limit by changing the simpleeval.POWER_MAX
module level value to whatever
is an appropriate value for you (and the hardware that you're running on) or if you want to
completely remove all limitations, you can set the s.operators[ast.Pow] = operator.pow
or make
your own function.
On my computer, 9**9**5
evaluates almost instantly, but 9**9**6
takes over 30 seconds.
Since 9**7
is 4782969
, and so over the POWER_MAX
limit, it throws a
``NumberTooHigh` exception for you. (Otherwise it would go on for hours, or until the computer
runs out of memory)
There are also limits on string length (100000 characters, MAX_STRING_LENGTH
).
This can be changed if you wish.
You can use python style if x then y else z
type expressions:
>>> simple_eval("'equal' if x == y else 'not equal'",
names={"x": 1, "y": 2})
'not equal'
which, of course, can be nested:
>>> simple_eval("'a' if 1 == 2 else 'b' if 2 == 3 else 'c'")
'c'
You can define functions which you'd like the expressions to have access to:
>>> simple_eval("double(21)", functions={"double": lambda x:x*2})
42
You can define "real" functions to pass in rather than lambdas, of course too, and even re-name them so that expressions can be shorter
>>> def double(x):
return x * 2
>>> simple_eval("d(100) + double(1)", functions={"d": double, "double":double})
202
Sometimes it's useful to have variables available, which in python terminology are called 'names'.
>>> simple_eval("a + b", names={"a": 11, "b": 100})
111
You can also hand the handling of names over to a function, if you prefer:
>>> def name_handler(node):
return ord(node.id[0].lower(a))-96
>>> simple_eval('a + b', names=name_handler)
3
That was a bit of a silly example, but you could use this for pulling values from a database or file, say, or doing some kind of caching system.
Rather than creating a new evaluator each time, if you are doing a lot of evaluations, you can create a SimpleEval object, and pass it expressions each time (which should be a bit quicker, and certainly more convienient for some use cases):
s = SimpleEval()
s.eval("1 + 1")
# and so on...
You can assign / edit the various options of the SimpleEval
object if you want to.
Either assign them during creation (like the simple_eval
function)
s = SimpleEval(functions={"boo": boo})
or edit them after creation:
s.names['fortytwo'] = 42
this actually means you can modify names (or functions) with functions, if you really feel so inclined:
s = SimpleEval()
def set_val(name, value):
s.names[name.value] = value.value
return value.value
s.functions = {'set': set_val}
s.eval("set('age', 111)")
Say. This would allow a certain level of 'scriptyness' if you had these evaluations happening as callbacks in a program. Although you really are reaching the end of what this library is intended for at this stage.
The library supports both python 2 and 3 using the 2to3 converter.
Please read the test_simpleeval.py
file for other potential gotchas or details. I'm very happy to accept pull requests, suggestions, or other issues. Enjoy!