Exiting program for any input that is not a list

Question

My goal is to create a little program that converts angle from radiant to degree and vice-versa. I need the program to close with no error message from python if the user enters the information to convert in the wrong format.

After assigning the variable ‘angle’ to both values of the input. The angle variable becomes a list type. In norther to exit program with no error message I write this: 'if angle is not list():break'.

The problem is when I do that it exits the program for any type of command entered as an input.

here is my code:

import numpy as np

while 1:

    angle=input("Please enter the angle you want to convert,\n\n"\
    "If you wish to convert degrees in radiant or vise-versa,\n"\
    "follow this format: 'angle/D or R'").split('/')


    if angle is not list():break


    angle[0]=float(angle[0])
    radiant= (angle[0]*(np.pi))/180
    degre=((angle[0]*180)/np.pi)

    if (angle[0]>=0 or angle[0]<=360) and angle[1] is 'D' :
        print(radiant,'radiants')
    elif angle[1] is 'R':
        print(degre,'degrés')
    else:break

Check this out: http://stackoverflow.com/questions/2225038/determine-the-type-of-an-object — mechanical_meat, Jun 17 '16 at 16:25
You might want to look at `isinstance` https://docs.python.org/2/library/functions.html#isinstance — Alex Huszagh, Jun 17 '16 at 16:25
`if angle is not list():` will *absolutely never evaluate truthy*, and it's not clear why you thought it would. It's creating a brand new object and asking if it's the same object as a different, pre-existing object. It's not. — jonrsharpe, Jun 17 '16 at 16:28
Understanding that python see’s ‘list() ‘ in this case as an object clarifies why it didn’t work. The isinstance() fonction is exactly what I needed. — B.gho, Jun 17 '16 at 22:43

Sreyantha Chary · Accepted Answer · 2016-06-17T16:49:01.993

You can use isinstance(angle, list) to check if it is a list. But it won't help you achieve what you really want to do.

The following code will help you with that.

question = """Please enter the angle you want to convert.
If you wish to convert degree in radiant or vice-versa.
Follow this format: 'angle/D or R'
"""

while 1:
    angle=input(question).split('/')

    if not isinstance(angle, list): break # This will never happen
    # It will never happen because string.split() always returns a list

    # Instead you should use something like this:
    if len(angle) != 2 or angle[1] not in ['D', 'R']:
        break

    try:
        angle[0]=float(angle[0])
    except ValueError:
        break

    if (angle[0]>=0 or angle[0]<=360) and angle[1] is 'D':
        # You could also improve this by taking modulo 360 of the angle.
        print((angle[0]*np.pi)/180, 'radiants')
    else:
        # Just an else is enough because we already checked that angle[1] is either D or R
        print((angle[0]*180)/np.pi, 'degrees')

score 0 · Answer 2 · answered Jun 17 '16 at 16:29

What you want:

if not isinstance(angle, list): break

What you've done: if angle is not list():break will always evaluate to True as no object will ever have the same identity as the list list(); since is is a check for identity.

Even this:

>>> list() is not list()
True

score 0 · Answer 3 · answered Jun 17 '16 at 17:08

break statements are used to get out of for and while loops. Try using a while loop after the input statement to evaluate the input. Use a possible set as a conditional. You do not need to break from an if statement because it will just be bypassed if the the conditional is not met.
Sometimes you might see an if statement followed by a break statement. The break statement, however, is not breaking from the if statement. It is breaking from a previous for or while loop.

Exiting program for any input that is not a list

3 Answers3