我的部分代码不会显示在shell中。 （if / elif语句）

Question

I'm new to Python. When I run this code, the last part where the results should be calculated is not shown on shell. Can someone tell me how should I fix this? The last part seems a little bit awkward, but I have no idea how to convert str into operations.

# Set variables
opList = ["plus", "minus", "times", "divided-by", "equals"]

# Instrution
print("Intructions: Please enter + as plus, - as minus, * as times and / as divided-by.")

# Read user's equation as a string
equation = input("\nPlease, enter your equation by following the syntax expressed above: ")

# Echo to the screen what the user has entered
print('The equation you entered is "%s".' %equation)

# Parse the equation into a list
theParts = equation.split() # default is whitespace

# print("Here is a list containing the operands and operator of the equation: ", theParts) # For debugging purposes

if len(theParts) == 0 :
    print("\nHave you simply pressed the Enter key? Please, enter an equation next time! :)")

elif len(theParts) == 1 :
    print("\nThis is not a equaltion so it cannot be calculated. Please, enter an equation next time! :)")  

elif len(theParts) == 2 :
    print("\nThis is not a equaltion so it cannot be calculated. Please, enter an equation next time! :)") 

elif len(theParts) == 3 :
    print("\nThe equation entered by the user is %s %s %s." %(theParts[0], theParts[1], theParts[2]))
                    if theParts[1] is str("plus"):
                        theAnswer == theParts[0] + theParts[2]
                        print('The anwser of the input equation is "%i".' %theAnswer)

                    elif theParts[1] is str("minus"):
                        theAnswer == theParts[0] - theParts[2]
                        print('The anwser of the input equation is "%i".' %theAnswer)

                    elif theParts[1] is str("times"):
                         theAnswer == theParts[0] * theParts[2]
                         print('The anwser of the input equation is "%i".' %theAnswer)

                    elif theParts [1] is str("divided-by"):
                         theAnswer == theParts[0] / theParts[2]
                         print('The anwser of the input equation is "%i".' %theAnswer)





print("\nBye!") 

Answer 1

Assuming you're using Python 2.x, the main issue that's getting you hung up is that you're using input instead of raw_input . input will evaluate your equation and use the evaluation in your program, whereas raw_input will get exactly what the user types as a string. For example:

input

# what the user types
3 + 7
# what the program gets as an integer
10

raw_input

# what the user types
3 + 7
# what the program gets as a string
"3 + 7"

No matter what version of Python you're using, you'll have to fix the following:

Indentation

You'll need to indent the code for your case where theParts has three integers so it executes only then. Otherwise it will execute no matter what and give you an array out of bounds error or the you'll get an indentation formatting error.

Testing string equality

Rather than use is str("[string]") simply use == . Don't try to over-complicate things.

Strings vs. Numbers

In order to do math, you'll have to convert your strings to numbers. You can do that using something like int("5") which is equal to 5 (integer).

Example Code

# case 3
elif len(theParts) == 3:
    # do stuff
    if "plus" == theParts[1]:
        theAnswer = int(theParts[0]) + int(theParts[2])

Answer 2

Assuming you are using a modern Python, input is actually the correct function to use (there is no raw_input in 3.x, and input is always a string). But, as mentioned, there are other problems.

Here's a version with some corrections.

# Instruction 
# I changed the instructions to make them more precise and get rid of the unneccesarily awkward operators
print("Instructions: Please enter a simple equation of the form 'integer [operator] integer', where [operator] is '+', '-', '*' or '/'.")
print("Instructions: Make sure to put spaces between each element.")

# Read user's equation as a string
equation = input("\nPlease, enter your equation by following the syntax expressed above: ")

# Echo to the screen what the user has entered
print('The equation you entered is "%s".' % equation)

# Parse the equation into a list
theParts = equation.split() # default is whitespace

# print("Here is a list containing the operands and operator of the equation: ", theParts) # For debugging purposes

if len(theParts) == 0 :
    print("\nHave you simply pressed the Enter key? Please, enter an equation next time! :)")
# Since the two conditions warranted the same response, I just condensed them.
elif len(theParts) == 1 or len(theParts) == 2:
    print("\nThis is not a equaltion so it cannot be calculated. Please, enter an equation next time! :)")  

elif len(theParts) == 3 :
    #print("\nThe equation entered by the user is %s %s %s." % (theParts[0], theParts[1], theParts[2]))
    # I set the answer before the conditions to a float, so division can result in more meaningful answers
    theAnswer = 0.0
    # I cast the input strings to integers
    p1 = int(theParts[0])
    p2 = int(theParts[2])
    if theParts[1] is str("+"):
        theAnswer = p1 + p2
    elif theParts[1] is str("-"):
        theAnswer = p1 - p2
    elif theParts[1] is str("*"):
        theAnswer = p1 * p2
    elif theParts [1] is str("/"):
        theAnswer = p1 / p2
    print('The anwser of the input equation is "{}".'.format(theAnswer))



print("\nBye!")

If you wanted division to have a more school book response, with a quotient and remainder, then instead of p1 / p2, you should use divmod(p1, p2) and parse out the two parts in your printed response.