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SymPy Package to bring Python's Sympy functionality into Julia via PyCall

SymPy (http://sympy.org/) is a Python library for symbolic mathematics.

With the excellent PyCall package of julia, one has access to the many features of the SymPy library from within a Julia session.

This SymPy package provides a light interface for the features of the SymPy library that makes working with SymPy objects a bit easier.

The documentation inludes an introduction document and a version of the SymPy tutorial translated from the Python syntax into Julia.

Installation

To use this package, both Python and its SymPy library must be installed on your system. If PyCall is installed using Conda (which is the default if no system python is found), then the underlying SymPy library will be installed via Conda when the package is first loaded. Otherwise, installing both Python and the SymPy library (which also requires mpmath) can be done by other means. In this case, the Anaconda distribution is suggested, as it provides a single installation of Python that includes SymPy and many other scientific libraries that can be profitably accessed within Julia via PyCall. (Otherwise, install Python then download the SymPy library from https://github.com/sympy/sympy/releases and install.)

To upgrade the underlying sympy library, which has new releases at a rate similar to Julia, when installed with Conda, the following commands are available:

using Pkg
Pkg.add("Conda") #  if needed
using Conda
Conda.update()

The PyCall interface to SymPy

The only point to this package is that using PyCall to access SymPy is somewhat cumbersome. The following is how one would define a symbolic value x, take its sine, then evaluate the symboic expression for x equal pi, say:

using PyCall
sympy = pyimport("sympy")  #
x = sympy.Symbol("x")      # PyObject x
y = sympy.sin(x)           # PyObject sin(x)
z = y.subs(x, sympy.pi)    # PyObject 0
convert(Float64, z)        # 0.0

The sympy object imported on the second line provides the access to much of SymPy's functionality, allowing access to functions (sympy.sin), properties, modules (sympy), and classes (sympy.Symbol, sympy.Pi). The Symbol and sin operations are found within the imported sympy module and, as seen, are referenced with Python's dot call syntax, as implemented in PyCall through a specialized getproperty method.

SymPy's functionality is also found through methods bound to an object of a certain class. The subs method of the y object is an example. Such methods are also accessed with Python's dot-call syntax. The call above substitutes a value of sympy.pi for the symbolic variable x. This leaves the object as a PyObject storing a number which can be brought back into julia through conversion, in this case through an explicit convert call.

Alternatively, PyCall now has a * method, so the above could also be done with:

x = sympy.Symbol("x")
y = sympy.pi * x
z = sympy.sin(y)
convert(Float64, z.subs(x, 1))

With the SymPy package this gets replaced by a more julian syntax:

using SymPy
x = symbols("x")		       # or   @vars x, Sym("x"), or  Sym(:x)
y = sin(pi*x)
y(1)                           # Does y.subs(x, 1). Use y(x=>1) to be specific as to which symbol to substitute

The object x we create is of type Sym, a simple proxy for the underlying PyObject. The package overloads the familiar math functions so that working with symbolic expressions can use natural julia idioms. The final result here is a symbolic value of 0, which prints as 0 and not PyObject 0. To convert it into a numeric value within Julia, the N function may be used, which acts like the float conversion, only there is an attempt to preserve the variable type.

(There is a subtlety, the value of pi here (an Irrational in Julia) is converted to the symbolic PI, but in general won't be if the math constant is coerced to a floating point value before it encounters a symbolic object. It is better to just use the symbolic value PI, an alias for sympy.pi used above.)

---

SymPy has a mix of function calls (as in sin(x)) and method calls (as in y.subs(x,1)). The function calls are from objects in the base sympy module. When the SymPy package is loaded, in addition to specialized methods for many generic Julia functions, such as sin, a priviledged set of the function calls in sympy are imported as generic functions narrowed on their first argument being a symbolic object, as constructed by Sym or symbols. (Calling import_from(sympy) will import all the function calls.)

The basic usage follows these points:

• generic methods from Julia and imported functions in the sympy namespace are called through fn(object)

• SymPy methods are called through Python's dot-call syntax: object.fn(...)

• Contructors, like sympy.Symbol, and other non-function calls from sympy are qualified with sympy.Constructor(...). Such qualified calls are also useful when the first argument is not symbolic.

So, these three calls are different,

sin(1), sin(Sym(1)), sympy.sin(1)

The first involves no symbolic values. The second and third are related and return a symbolic value for sin(1). The second dispatches on the symbolic argument Sym(1), the third has no dispatch, but refers to a SymPy function from the sympy object. Its argument, 1, is converted by PyCall into a Python object for the function to process.

In the initial example, slightly rewritten, we could have written:

x = symbols("x")
y = sin(pi*x)
y.subs(x, 1)

The first line calls a provided alias for sympy.symbols which is defined to allow a string (or a symbol) as an argument. The second, dispatches to sympy.sin, as pi*x is symbolic-- x is, and multiplication promotes to a symbolic value. The third line uses the dot-call syntax of PyCall to call the subs method of the symbolic y object.

Not illustrated above, but classes and other objects from SymPy are not brought in by default, and can be accessed using qualification, as in sympy.Function (used to define symbolic functions).