Print substituted expression without numerical evaluation

问题

Is it possible to print a substitute SymPy expression without calculation? I want to print both substituted express and result.

e.g.

x = Symbol('x')
expr = x**2
pprint(expr) # this prints expression
result = expr.subs({x:2}) 
print(result) # this print result 4

How can I print the "middle result", the expression 2**2?

回答1:

You can pass in UnevaluatedExpr for this purpose, as shown below:

result = expr.subs(x, UnevaluatedExpr(2))
print(result)   # prints 2**2 
result = result.doit()
print(result)   # prints 4

Documentation: Prevent expression evaluation

来源：https://stackoverflow.com/questions/45331069/print-substituted-expression-without-numerical-evaluation