sorting with a condition Grade Letters

I am trying to sort grade letters where B+ must be before B etc.

import random
grades = ['A', 'A-', 'B+', 'B', 'B-', 'C+', 'C', 'C-', 'D+', 'D', 'F']
random.shuffle(grades)
print(grades)

['C', 'B', 'F', 'B+', 'D+', 'A', 'A-', 'C-', 'C+', 'B-', 'D']

Using sorted function brings B before B+, etc.
I used the key with sorted function but I am not aware how exactly to use it!

sorted(grades, key=lambda x: x.endswith('+'))

Any help is appreciated.

I searched a lot for a similar question without success!

1. Use the int value from bool,

• if there is + it'll be -1

• if there is - it'll be 1

• if none of, it'll be 0

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  grades.sort(key=lambda x: (x[0], - x.endswith('+') + x.endswith('-')))
  

2. Using a mapping dict to convert sign to a number to be sorted :

    lettermap = {'-': 2, '': 1, '+': 0}
    grades.sort(key=lambda x: (x[0], lettermap[x[1:]])) # tuple to have multiple criterias
    print(grades) # ['A', 'A-', 'B+', 'B', 'B-', 'C+', 'C', 'C-', 'D+', 'D', 'F']
   

• Same with inline dict

        grades.sort(key=lambda x: (x[0], {'-': 2, '': 1, '+': 0}[x[1:]]))
        grades.sort(key=lambda x: (x[0], {'-': 2, '+': 0}.get(x[1:], 1)))
  

The key function maps the elements to real numbers, the sort function will then sort the elements in your list by those values. In order to produce such keys for the sort function, try mapping the letters to integers e.g. with ord and adjust those values to decimals if there is a sign after!

def keyfunc(grade):
    value = ord(grade[0]) #every letter gets mapped to a number
    if len(grade)>1:
        if grade[1] == '+': #plus grades rank higher -> subtract a little
            value -= 0.3
        else:
            value += 0.3 #for minus , add a little
    return value
sorted(grades, key=keyfunc)

this produces:

['A', 'A-', 'B+', 'B', 'B-', 'C+', 'C', 'C-', 'D+', 'D', 'F']

You could use dict.get and supply a default return:

sorted(grades, key=lambda g: (g[0], {'-': 2, '+': 0}.get(g[-1], 1)))

Alternatively, you could map each of the letter grades directly to a numerical value and sort based on that:

mapping = {'A': 10, 'A-': 9, 'B+': 8, 'B': 7, 'B-': 6, 'C+': 5, 'C': 4, 'C-': 3, 'D+': 2, 'D': 1, 'F': 0}

sorted(grades, key=lambda x: mapping[x.upper()], reverse=True)
# ['A', 'A-', 'B+', 'B', 'B-', 'C+', 'C', 'C-', 'D+', 'D', 'F']

Enhanced version of @peer's answer. More case handling like 'A+' > 'b-' and grades such as "A++".

def grade_valuator(grade):
    postgrade = grade.upper()
    value = 0
    i = 2
    while(len(postgrade) > 0):
        if postgrade[0] == '+':
            value -= .5 ** i
            i += 1
        elif postgrade[0] == '-':
            value += .5 ** i
            i += 1
        elif postgrade[0].isalnum():
            value += ord(postgrade[0])
        postgrade = postgrade[1:]
    return value

Code use examples:

grades = ['+d', 'b+', 'A', 'A-', 'C-', 'C', 'A+', 'C++', 'b-', 'A++', 'D', 'E', 'B', 'C+', 'C+-', 'F', '--C']


sorted(grades, key=grade_valuator) #Get a new sorted copy of the list
#['A++', 'A+', 'A', 'A-', 'b+', 'B', 'b-', 'C++', 'C+', 'C+-', 'C', 'C-', '--C', '+d', 'D', 'E', 'F']

print(grades)
#['+d', 'b+', 'A', 'A-', 'C-', 'C', 'A+', 'C++', 'b-', 'A++', 'D', 'E', 'B', 'C+', 'C+-', 'F', '--C']

grades.sort(key=grade_valuator) #Sort the list in-place
print(grades)
#['A++', 'A+', 'A', 'A-', 'b+', 'B', 'b-', 'C++', 'C+', 'C+-', 'C', 'C-', '--C', '+d', 'D', 'E', 'F']