Evaluate Mathematical Expressions
in Python

Python is an interpreted language and it does have an eval() function that makes evaluating a Python expression easier. Unlike compiled languages, in Python, we do not need to re-invent the wheel to write a math parser for this purpose. Parsing of the expression is done by Python at runtime.

If you do not care about the AST (Abstract Syntax Tree) of the expression, you can simply pass a string expression to the eval() function to get the value. The expression syntax will be limited to what the Python language supports.

For example:

eval(�1+3�)

However, in many situations, you need to collect input from the user and evaluate it as a mathematical expression. If you have a web application running on the server side, a malicious user can type a function call that can have undesired side effects when evaluated with the eval() Python function. To guard for this security hole, you need a way to limit what functions and variables the user can actually use.

PyMathParser - Evaluate Math Expression

Bestcode is pleased to introduce PyMathParser Python class that simplifies the math expression evaluation for you. PyMathParser maintains a list of permitted functions and variables to limit Python�s eval() function to these symbols only.

The Math Parser Component is also available on many platforms/languages such as:

• bcParser.NET for .NET, C#, Visual Basic
• JbcParser for Java
• TbcParser for Delphi, C++ Builder
• bcParserGo for Go
• bcParserObjC for Objective C
• bcParserCPP for vanilla C++ (Visual C++, GNU C++, Windows, Linux, Mac)
• Math Parser for PHP
• Math Parser for Python - Free download

Example of Evaluating a Formula using PyMathParser

Here is the code for PyMathParser that is used to evaluate a mathematical expression in a safe way:

from math import * class PyMathParser(object):     '''     Mathematical Expression Evaluator class.     You can set the expression member, set the functions, variables and then call     evaluate() function that will return you the result of the mathematical expression     given as a string.     '''         '''     Mathematical expression to evaluate.     '''     expression = ''         '''     Dictionary of functions that can be used in the expression.     '''     functions = {'__builtins__':None};         '''     Dictionary of variables that can be used in the expression.     '''     variables = {'__builtins__':None};     def __init__(self):         '''         Constructor         '''         def evaluate(self):         '''         Evaluate the mathematical expression given as a string in the expression member variable.                 '''         return eval(self.expression, self.variables, self.functions);     def addDefaultFunctions(self):         '''         Add the following Python functions to be used in a mathemtical expression:         acos         asin         atan         atan2         ceil         cos         cosh         degrees         exp         fabs         floor         fmod         frexp         hypot         ldexp         log         log10         modf         pow         radians         sin         sinh         sqrt         tan         tanh         '''         self.functions['acos']=acos         self.functions['asin']=asin         self.functions['atan']=atan         self.functions['atan2']=atan2         self.functions['ceil']=ceil         self.functions['cos']=cos         self.functions['cosh']=cosh         self.functions['degrees']=degrees         self.functions['exp']=exp         self.functions['fabs']=fabs         self.functions['floor']=floor         self.functions['fmod']=fmod         self.functions['frexp']=frexp         self.functions['hypot']=hypot         self.functions['ldexp']=ldexp         self.functions['log']=log         self.functions['log10']=log10         self.functions['modf']=modf         self.functions['pow']=pow         self.functions['radians']=radians         self.functions['sin']=sin         self.functions['sinh']=sinh         self.functions['sqrt']=sqrt         self.functions['tan']=tan         self.functions['tanh']=tanh             def addDefaultVariables(self):         '''         Add e and pi to the list of defined variables.         '''         self.variables['e']=e         self.variables['pi']=pi     def getVariableNames(self):         '''         Return a List of defined variables names in sorted order.         '''         mylist = list(self.variables.keys())         try:             mylist.remove('__builtins__')         except ValueError:             pass         mylist.sort()         return mylist     def getFunctionNames(self):         '''         Return a List of defined function names in sorted order.         '''         mylist = list(self.functions.keys())         try:             mylist.remove('__builtins__')         except ValueError:             pass         mylist.sort()         return mylist

Simple Python Unit Test for PyMathParser

Here is the unit test that shows how to invoke PyMathParser to evaluate a user defined formula given as a string:

''' Unit tests for PyMathParser. ''' import unittest from bcmathparser import MathParser from math import * class Test(unittest.TestCase):     def setUp(self):         self.mathparser = MathParser.PyMathParser()         self.mathparser.addDefaultFunctions()         self.mathparser.addDefaultVariables()         self.mathparser.variables['x'] = 1.5;         self.mathparser.variables['y'] = 2.0;     def tearDown(self):         pass     def testBasic(self):         self.mathparser.expression = '1+ 2'         val = self.mathparser.evaluate()         self.assertEquals(3, val)         print val     def testEvalExpression(self):         self.mathparser.expression = '1+ 2+sin (x+ y)/e+pi'         print self.mathparser.evaluate()     def testNegativeFailNoFunction(self):         self.mathparser.expression = '1+2+sin( x + y)/abc(x)'         try:             print self.mathparser.evaluate()         except NameError as (err):             if (err.message!="name 'abc' is not defined"):                 raise err             print 'Expected error: ', err #expected error: name 'abc' is not defined     def testFunctionNames(self):         print self.mathparser.getFunctionNames()     def testVariableNames(self):         print self.mathparser.getVariableNames() if __name__ == "__main__":     #import sys;sys.argv = ['', 'Test.testName']     unittest.main()

PyMathParser Class Documentation Generated by PyDoc

bcmathparser.MathParser

Mathematical Expression Evaluator class for Python.
You can set the expression member, set the functions, variables and then call
evaluate() function that will return you the result of the mathematical expression
given as a string. 
 
The user is granted rights to user, distribute and/or modify the source code 
as long as this notice is shipped with it.
 
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Classes
		__builtin__.object PyMathParser   class PyMathParser(__builtin__.object)     Mathematical Expression Evaluator class. You can set the expression member, set the functions, variables and then call evaluate() function that will return you the result of the mathematical expression given as a string.     Methods defined here: __init__(self) Constructor addDefaultFunctions(self) Add the following Python functions to be used in a mathemtical expression: acos asin atan atan2 ceil cos cosh degrees exp fabs floor fmod frexp hypot ldexp log log10 modf pow radians sin sinh sqrt tan tanh addDefaultVariables(self) Add e and pi to the list of defined variables. evaluate(self) Evaluate the mathematical expression given as a string in the expression member variable. getFunctionNames(self) Return a List of defined function names in sorted order. getVariableNames(self) Return a List of defined variables names in sorted order. Data descriptors defined here: __dict__ dictionary for instance variables (if defined) __weakref__ list of weak references to the object (if defined) Data and other attributes defined here: expression = '' functions = {'__builtins__': None} variables = {'__builtins__': None}

Functions
		acos(...) acos(x)   Return the arc cosine (measured in radians) of x. acosh(...) acosh(x)   Return the hyperbolic arc cosine (measured in radians) of x. asin(...) asin(x)   Return the arc sine (measured in radians) of x. asinh(...) asinh(x)   Return the hyperbolic arc sine (measured in radians) of x. atan(...) atan(x)   Return the arc tangent (measured in radians) of x. atan2(...) atan2(y, x)   Return the arc tangent (measured in radians) of y/x. Unlike atan(y/x), the signs of both x and y are considered. atanh(...) atanh(x)   Return the hyperbolic arc tangent (measured in radians) of x. ceil(...) ceil(x)   Return the ceiling of x as a float. This is the smallest integral value >= x. copysign(...) copysign(x,y)   Return x with the sign of y. cos(...) cos(x)   Return the cosine of x (measured in radians). cosh(...) cosh(x)   Return the hyperbolic cosine of x. degrees(...) degrees(x) -> converts angle x from radians to degrees exp(...) exp(x)   Return e raised to the power of x. fabs(...) fabs(x)   Return the absolute value of the float x. factorial(...) factorial(x) -> Integral   Find x!. Raise a ValueError if x is negative or non-integral. floor(...) floor(x)   Return the floor of x as a float. This is the largest integral value <= x. fmod(...) fmod(x,y)   Return fmod(x, y), according to platform C.  x % y may differ. frexp(...) frexp(x)   Return the mantissa and exponent of x, as pair (m, e). m is a float and e is an int, such that x = m * 2.**e. If x is 0, m and e are both 0.  Else 0.5 <= abs(m) < 1.0. fsum(...) sum(iterable)   Return an accurate floating point sum of values in the iterable. Assumes IEEE-754 floating point arithmetic. hypot(...) hypot(x,y)   Return the Euclidean distance, sqrt(x*x + y*y). isinf(...) isinf(x) -> bool Checks if float x is infinite (positive or negative) isnan(...) isnan(x) -> bool Checks if float x is not a number (NaN) ldexp(...) ldexp(x, i) -> x * (2**i) log(...) log(x[, base]) -> the logarithm of x to the given base. If the base not specified, returns the natural logarithm (base e) of x. log10(...) log10(x) -> the base 10 logarithm of x. log1p(...) log1p(x)   Return the natural logarithm of 1+x (base e).       The result is computed in a way which is accurate for x near zero. modf(...) modf(x)   Return the fractional and integer parts of x.  Both results carry the sign of x and are floats. pow(...) pow(x,y)   Return x**y (x to the power of y). radians(...) radians(x) -> converts angle x from degrees to radians sin(...) sin(x)   Return the sine of x (measured in radians). sinh(...) sinh(x)   Return the hyperbolic sine of x. sqrt(...) sqrt(x)   Return the square root of x. tan(...) tan(x)   Return the tangent of x (measured in radians). tanh(...) tanh(x)   Return the hyperbolic tangent of x. trunc(...) trunc(x:Real) -> Integral   Truncates x to the nearest Integral toward 0. Uses the __trunc__ magic method.

Data
		e = 2.7182818284590451 pi = 3.1415926535897931

 

Download PyMathParser for FREE

You can download PyMathParser source code for free.

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