How to let the user reuse a result from the previous computation in a python calculator

Question

I am fairly new at python so I don't know much about it. I made a calculator and I want it to accept a:

ans()

input. Currently, there is a part that stops the program from executing an input if there is something other than [0-9 */-+] so it does not crash. How can I make

ans()

represent the output of the equation last entered so I can enter something like this:

>> 8*8 #last input
64 #last output
>> ans()*2 #current input
128 # current output

Hopefully I explained everything correctly and here is my code:

valid_chars = "0123456789-+/* \n";
while True:
x = "x="
y = input(" >> ")
x += y
if any(c not in valid_chars for c in y):
    print("WARNING: Invalid Equation")
    continue
try:
    exec(x)
except (SyntaxError, ZeroDivisionError):
    print ("WARNING: Invalid Equation")
else:
    print(x)

Update: I added several lines that were recommended in the answers but it would not run (I also fixed the indents):

valid_chars = "0123456789-+/* \n";
while True:
    x = "x="
    y = input(" >> ")
    x += y
    def ans():
        return _
    def ans():
        try:
            return _
    except NameError:
        return 0 # appropriate value
    if any(c not in valid_chars for c in y):
        print("WARNING: Invalid Equation")
        continue
    try:
        exec(x)
    except (SyntaxError, ZeroDivisionError):
        print ("WARNING: Invalid Equation")
    else:
        print(x)

Error: "Unexpected Indent" for except NameError

What did I do wrong and how can I fix this? Thanks

Answer 1

If you want to use your program in the python interpreter, you can use a simple trick. The interpreter stores the last output in the special variable _:

>>> def ans():
...     return _
... 
>>> 8 * 8
64
>>> ans() * 8
512
>>> def ans():
...     return _
... 
>>> 8 * 8
64
>>> ans() * 2
128
>>> 

Just remember, that _ can raise a NameError if there hasn't been any output so far. You can handle it with something like that:

>>> def ans():
...     try:
...         return _
...     except NameError:
...         return 0 # appropriate value
... 

Answer 2

In the Python REPL loop _ represents the last result; however, this may not be the case in your calculator.

Try something like

valid_chars = "0123456789-+/*()ans \n";
result = None

...

result = eval(x.replace('ans()',str(result))

You probably want something better than valid_chars though, and parse for correct expressions. Also, as an aside, what you are evaluating are expressions not equations.

Hope this helps.

Answer 3

This:

valid_chars = "0123456789-+/* \n"
while True:
    x = "x="
    y = input(" >> ")
    x += y
    def ans():
        return z
    def ans():
        try:
            return z
    except NameError:
        return 0 # appropriate value
    if any(c not in valid_chars for c in y):
        print("WARNING: Invalid Equation")
        continue
    try:
        exec(x)
    except (SyntaxError, ZeroDivisionError):
        print ("WARNING: Invalid Equation")
    else:
        z = x
        print(x)