**SymPy** (http://www.sympy.org) is a Python library for **symbolic math**.

In symbolic math, symbols are used to represent mathematical expressions. An example of a symbolic math expression is below:

In the expression above, we have the variables $x$, $y$ and $z$.

If we define a second symbolic math expression as:

We can substitute in $a + b$ for $x$.

The resulting expression is:

Solving for $y$ in terms of $a$,$b$ and $z$, results in:

$$ y = \sqrt{z - a^{2} - 2ab - b^{2}} $$If we have numerical values for $z$, $a$ and $b$, we can use Python to calculate the value of $y$.

However, if we don't have numerical values for $z$, $a$ and $b$, Python can also be used to rearrange terms of the expression and solve for the variable $y$ in terms of the other variables $z$, $a$ and $b$. Working with mathematical symbols in a programmatic way, instead of working with numerical values in a programmatic way, is called symbolic math.

**SymPy** is a Python library for working with symbolic math. Before **SymPy** can be used, it needs to be installed. The installation of **Sympy** is accomplished using the **Anaconda Prompt** (or a terminal and **pip**) with the command:

```
> conda install sympy
```

**SymPy** is included in the Anaconda distribution of Python. If you have the full Anaconda distribution, you will be notified that the **SymPy** library is already installed.

## Defining Variables in SymPy¶

To define symbolic math variables with **SymPy**, first import the `symbols()`

function from the **SymPy** module:

```
from sympy import symbols
```

Symbolic math variables are declared using **SymPy's** `symbols()`

function. Note, the arguments passed to the `symbols()`

function (symbol names) are separated by a space, no comma, and surrounded by quotes. The output of the `symbols()`

function are **SymPy** symbols objects. These output objects are separated by commas with no quotation marks.

```
x, y = symbols('x y')
```

Now that the symbols `x`

and `y`

are instantiated, a *symbolic math expression* using `x`

and `y`

can be created.

A symbolic math expression is a combination of symbolic math variables with numbers and mathematical operators, such as `+`

,`-`

,`/`

and `*`

. The standard Python rules for working with numbers apply in **SymPy** symbolic math expressions.

```
expr = 2*x + y
```

Use the `.subs()`

method to insert a numerical value into a symbolic math expression. The first argument of the `.subs()`

method is the symbols object (the variable) and the second argument is the numerical value. In the expression above:

If we substitute:

$$ x = 2 $$The resulting expression is:

$$ 2(2) + y $$$$ 4 +y $$```
expr.subs(x, 2)
```

The `.subs()`

method does not replace variables in place, `.subs()`

only completes a one-time substitution. If we call `expr`

after the `.subs()`

method is applied, the original `expr`

expression is returned.

```
expr
```

In order to make the substitution permanent, a new expression object needs to be assigned to the output of the `.subs()`

method.

```
expr = 2*x + y
expr2 = expr.subs(x, 2)
expr2
```

**SymPy** variables can also be substituted into **SymPy** expressions

```
x, y, z = symbols('x y z')
expr = 2*x + y
expr2 = expr.subs(x, z)
expr2
```

More complex substitutions can also be completed. Consider the following:

$$ 2x + y $$Substitute in:

$$ y = 2x^2 + z^{-3} $$Results in:

$$ 2x + 2x^2 + z^{-3} $$```
x, y, z = symbols('x y z')
expr = 2*x + y
expr2 = expr.subs(y, 2*x**2 + z**(-3))
expr2
```

A more practical example could involve a large expression and several variable substitutions.

$$ n_0e^{-Q_v/RT} $$$$ n_0 = 3.48 \times 10^{-6} $$$$ Q_v = 12,700 $$$$ R = 8.31 $$$$ T = 1000 + 273 $$```
from sympy import symbols, exp
n0, Qv, R, T = symbols('n0 Qv R T')
expr = n0*exp(-Qv/(R*T))
```

Multiple **SymPy** `subs()`

methods can be chained together to substitue multiple variables in one line of code.

```
expr.subs(n0, 3.48e-6).subs(Qv,12700).subs(R, 8031).subs(T, 1000+273)
```

To evaluate an expression as a floating point number (get a numerical answer out), use **Sympy's** `.evalf()`

method.

```
expr2 = expr.subs(n0, 3.48e-6).subs(Qv,12700).subs(R, 8031).subs(T, 1000+273)
expr2.evalf()
```

## Defining Equations in Sympy¶

We can define *equations* in **SymPy** using symbolic math variables. *Equations* in **SymPy** are different than *expressions* in **SymPy**. An expression does not have equality. An equation has equality. An equation is equal to something.

```
from sympy import symbols, Eq, solve
```

```
x, y = symbols('x y')
```

**SymPy** equations are instantiated as an object of the `Eq`

class. After **SymPy** symbols are created, the symbols can be passed into an equation object. Let's create the equation:

```
eq1 = Eq(2*x - y - 1)
```

Now let's create a second equation:

$$ x + y - 5 = 0 $$```
eq2 = Eq(x + y - 5)
```

To solve the two equations for the two variables `x`

and `y`

, we'll use **SymPy's** `solve()`

function. The `solve()`

function takes two arguments, a tuple of the equations `(eq1, eq2)`

and a tuple of the variables to solve for `(x, y)`

.

```
sol = solve((eq1, eq2),(x, y))
sol
```

The **SymPy** solution object is a Python dictionary. The keys are the **SymPy** variable objects and the values are the numerical values these variables correspond to.

```
print(f'The solution is x = {sol[x]}, y = {sol[y]}')
```

## Summary¶

In this post, we looked at a Python package for symbolic math called **SymPy**. Using symbolic math, we can define expressions and equations exactly in terms of symbolic variables. We reviewed how to create a **SymPy** expression and substitue values and variables into the expression. Then we created to **SymPy** equation objects and solved two equations for two unknowns using **SymPy's** `solve()`

function.