Determine whether three sides form a valid triangle, and classify the triangle

Question

This is a my first Python program, and whilst I am new to Python I would like to keep in good practice

Below is a short program to work out what type a triangle is and if it makes a valid triangle

I have tried to use as little documention for this as possible to try and get used to how things work in Python so I can only imagine the mistakes I have made. However please do mention anything that should have be done better

# Determin if triangle is Scalene. Isosceles or equilateral
# Also works out if lengths can make a triangle
from decimal import *
getcontext().prec = 3
getcontext().rounding = ROUND_HALF_UP
#Needs to be divided to re-set decimal place I think
a = Decimal(input("Length of side a = ")) / 1
b = Decimal(input("Length of side b = ")) / 1
c = Decimal(input("Length of side c = ")) / 1
if a != b and b != c and a != c:
 print("This is a a Scalene triangle")
 triangle_type = 'Scalene'
elif a == b and c == b:
 print("This is an Equilateral triangle")
 triangle_type = 'Equilateral'
else:
 print("This is an Isosceles triangle")
 triangle_type = 'Isosceles'
def is_valid_triangle(a, b, c,triangle_type):
 if triangle_type == 'Equilateral':
 return True #all same lengths will be a valid triangle
 elif triangle_type == 'Isosceles' or triangle_type == 'Scalene':
 if a == b:
 return a + b > c
 elif b == c:
 return b + c > a
 elif a == c:
 return a + c > b
 else: #This will be the scalene triangle
 return a + b > c
 else:
 return False #Message is unclear as could be lengths are negitive or correct int type not used
print('Is this a valid triangle?', is_valid_triangle(a,b,c,triangle_type))

• πάντα ῥεῖ: "so I can only imagine the mistakes I have made" Does it work for your test cases?

Answer 1

Add a __name__ == "__main__" guard, and move the logic into a function separate from the I/O:

def triangle_type(a, b, c):
 '''Return a string indicating the type of triangle
 (Equilateral, Isosceles, Scalene, Impossible)
 '''
 # implementation here...
def main():
 getcontext().prec = 3
 getcontext().rounding = ROUND_HALF_UP
 #Needs to be divided to re-set decimal place I think
 a = Decimal(input("Length of side a = ")) / 1
 b = Decimal(input("Length of side b = ")) / 1
 c = Decimal(input("Length of side c = ")) / 1
 print(f"This is a {triangle_type(a, b, c)} triangle")
if __name__ == "__main__":
 main()

In the implementation, we can save a lot of "or" tests by sorting the lengths before we start:

a, b, c = sorted([a, b, c])
if a + b <= c:
 # N.B. automatically catches a < 0, since b <= c
 return 'Impossible'
if a != b != c:
 return 'Scalene'
elif a == c:
 return 'Equilateral'
else:
 return 'Isosceles'

---

Modified code

def triangle_type(a, b, c):
 '''
 Return a string indicating the type of triangle
 (Equilateral, Isosceles, Scalene, Impossible)
 '''
 a, b, c = sorted([a, b, c])
 if a + b <= c:
 return 'Impossible'
 if a != b != c:
 return 'Scalene'
 if a == c:
 return 'Equilateral'
 return 'Isosceles'
 
def main():
 a = input("Length of side a: ")
 b = input("Length of side b: ")
 c = input("Length of side c: ")
 print(f"({a}, {b}, {c}) is a {triangle_type(a, b, c)} triangle")
if __name__ == "__main__":
 main()

---

Further improvement

Use the doctest module to write the tests:

def triangle_type(a, b, c):
 '''
 Return a string indicating the type of triangle
 (Equilateral, Isosceles, Scalene, Impossible)
 >>> triangle_type(1, 1, 2)
 'Impossible'
 >>> triangle_type(-1, -1, -1)
 'Impossible'
 >>> triangle_type(1, 1.0, 1)
 'Equilateral'
 >>> triangle_type(1, 2, 2)
 'Isosceles'
 >>> triangle_type(2, 3, 2)
 'Isosceles'
 >>> triangle_type(2, 3, 4)
 'Scalene'
 '''
 a, b, c = sorted([a, b, c])
 if a + b <= c:
 return 'Impossible'
 if a != b != c:
 return 'Scalene'
 if a == c:
 return 'Equilateral'
 return 'Isosceles'
if __name__ == "__main__":
 import doctest
 doctest.testmod()

• Mr.Burns: Thanks for your answers, I have marked this one as accepted as it contains the more useful information, however both answers provide good points that I need to improve upon

Answer 2

A bug (or poorly-specified behaviour):

If we enter an invalid triangle (e.g. 1, 2, 4), the program reports that it's scalene, before telling us that it's not a valid triangle. That's a contradiction - if it's not a triangle, it cannot be a scalene triangle!

I recommend performing the is_valid_triangle() test first, and only continuing to classify the triangle if the test is successful.

And don't forget that valid triangles have three positive sides - any negative values should fail the validity test.

• sahasrara62: little suggestion to test validity of triangle, valid triangles have three positive sides and sum of two side is greater than 3rd side
• Toby Speight: @prashant, the posted code already covers the second part of that test; I'm just drawing attention to the missing part. I hope that's clear!
• sahasrara62: didn't went throuh the whole code, my mistake, thanks for commenting out