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Assigning a variable directly does not modify expressions that used the variable retroactively.
>>> from sympy import Symbol >>> x = Symbol('x') >>> y = Symbol('y') >>> f = x + y >>> x = 0 >>> f x + y 
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The subs() function in SymPy replaces all occurrences of first parameter with second. This function is useful if we want to evaluate a certain expression. For example, we want to calculate values of following expression by substituting a with 5.
subs() method, we can substitute all instances of a variable or expression in a mathematical expression with some other variable or expression or value. Parameters: variable – It is the variable or expression which will be substituted. substitute – It is the variable or expression or value which comes as substitute.
To evaluate a numerical expression into a floating point number, use evalf . SymPy can evaluate floating point expressions to arbitrary precision. By default, 15 digits of precision are used, but you can pass any number as the argument to evalf .
To substitute several values:
>>> from sympy import Symbol >>> x, y = Symbol('x y') >>> f = x + y >>> f.subs({x:10, y: 20}) >>> f 30 The command x = Symbol('x') stores Sympy's Symbol('x') into Python's variable x. The Sympy expression f that you create afterwards does contain Symbol('x'), not the Python variable x.
When you reassign x = 0, the Python variable x is set to zero, and is no longer related to Symbol('x'). This has no effect on the Sympy expression, which still contains Symbol('x').
This is best explained in this page of the Sympy documentation: http://docs.sympy.org/latest/gotchas.html#variables
What you want to do is f.subs(x,0), as said in other answers.
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