**assumptions in __new__ in Symbol class

[Abhishek K Das]

    def __new__(cls, name, **assumptions):
        """Symbols are identified by name and assumptions::


        >>> from sympy import Symbol
        >>> Symbol("x") == Symbol("x")
        True
        >>> Symbol("x", real=True) == Symbol("x", real=False)
        False


        """


        if assumptions.get('zero', False):
            return S.Zero
        is_commutative = fuzzy_bool(assumptions.get('commutative', True))
        if is_commutative is None:
            raise ValueError(
                '''Symbol commutativity must be True or False.''')

I was studying how declaration of Symbol works with the help of winpdb . `name` is for the name of symbol and `assumptions` are the assumptions associated with it .

What about the cls variable , I didn't understand it's use and what does assumptions.get('zero',False) mean ? 

[Abhishek K Das]

Along with this I needed to ask one more thing as for the definition of class Zero , there are methods where S.Zero is returned . Has the word Zero been hardcoded to 0 somewhere because

I don't understand how that returns value 0 . Similarly is the case with `One` also . 

[Aaron Meurer]

cls is the class that is being instantiated. It will usually be
Symbol, but if something subclasses Symbol, it will be that instead.
It is similar to self, except self is usually used for instances, not
classes.

Zero is the SymPy version of 0. If you type "0", that gives the Python
int(0). Zero prints as 0, and any time you combine another SymPy
operation with 0, it gets transformed to 0 (via sympify()). The
"Advanced Expression Manipulation" section of the tutorial explains
this in more detail.

Aaron Meurer

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