Expression Trees¶

What's an expression tree?

In [1]:

import pymbolic.primitives as p
x = p.Variable("x")
y = p.Variable("y")
x

Out[1]:

Variable('x')

Let's look what happens with a simple expression:

In [2]:

x+5

Out[2]:

Sum((Variable('x'), 5))

It does not get evaluated.

Let's look at its type and structure in more detail.

In [3]:

u = x+5

In [4]:

type(u)

Out[4]:

pymbolic.primitives.Sum

In [5]:

u.children

Out[5]:

(Variable('x'), 5)

OK, easy. What if we introduce a product?

In [6]:

u = x + 4*y
u

Out[6]:

Sum((Variable('x'), Product((4, Variable('y')))))

In [7]:

u.children[0]

Out[7]:

Variable('x')

In [8]:

u.children[1]

Out[8]:

Product((4, Variable('y')))

In [9]:

u.children[1].children[0]

Out[9]:

In [22]:

u.children[1].children[1]

Out[22]:

Variable('y')

This structure is a called a tree, because there is a root and branches.