def test_variable_replacement():
"""Test replace variables by values and symbols in expression."""
expr = demo_fall
vdict = Variable.__defaults__.copy()
vdict[Symbol('x')] = 1
assert replace_variables(expr, vdict) == \
Eq(demo_d, 4.9 * demo_fall.definition.t ** 2)
def test_substitution():
"""Test if .subs() method gives the same result as replace_variables."""
assert demo_fall.subs(demo_d, demo_d1) == \
replace_variables(demo_fall, {demo_d: demo_d1})