NameError: name 'a' is not defined pycharm

Your __init__ method needs to store a, b, and c as attributes of self:

def __init__(self, a, b, c):
    self.valido = None
    self.classificacao_lado = None  # Equilátero, Isósceles, Escaleno
    self.classificacao_angulo = None  # Retângulo, Obtusângulo, Acutângulo
    self.a, self.b, self.c = a, b, c

And then use self.a etc. to reference these values in your other methods.

def istriang(self):
    if self.a >= self.b + self.c or self.b >= self.a + self.c or self.c >= self.a + self.bb:
        self.valido = False
    else:
        self.valido = True

(there are other problems in this method, such as a missing else statement... I have taken the liberty of adding one)

Similar changes are needed in the other methods to refer to the sides of the triangle as attributes of the instance.

Try this:

class Triang:
    def __init__(self, a, b, c):
        self.valido = None        
        self.a = a
        self.b = b
        self.c = c

    def istriang(self):
        if self.a >= self.b+self.c or self.b >= self.a+self.c or self.c >= self.a+self.b:
            self.valido = False
        self.valido = True

    def classificar_lado(self):
        if self.a == self.b == self.c:
            return 'equilatero'
        if (self.a == self.b and self.a != self.c) or (self.a == self.c and self.a != self.b) or (self.b == self.c and self.b != self.a):
            return 'isosceles'
        else:
            return 'escaleno'

    def classificar_angulo(self):
        if self.a**2 == self.b**2 + self.c**2:
            return 'retangulo'
        if self.a**2 > self.b**2 + self.c**2:
            return  'obtusangulo'
        if self.a ** 2 < self.b ** 2 + self.c ** 2:
            return 'acutangulo'

A, B, C = [float(item) for item in input().split()]  # 5 5 5
triangulo = Triang(A, B, C)
lado = triangulo.classificar_lado()
angulo = triangulo.classificar_angulo()
print(lado)
print(angulo)

Can I suggest a different way? Having all those selfs in the code makes it very hard to read plus tedious to write and to change.

You could instead have istriang, classificar_lado and classificar_angulo as regular free functions (not associated with any class), pass a, b, and c into the functions and return the result.

Here is the code I have suggested. You may notice that its simpler to read and your functions are more generally useful, as they are not associated with any class.

def istriang(a, b, c):
    if a >= b+c or b >= a+c or c >= a+b:
        return False
    return True


def classificar_lado(a, b, c):
    if a == b == c:
        return 'equilatero'
    if (a == b and a != c) or (a == c and a != b) or (b == c and b != a):
        return 'isosceles'
    else:
        return 'escaleno'


def classificar_angulo(a, b, c):
    if a**2 == b**2 + c**2:
        return 'retangulo'
    if a**2 > b**2 + c**2:
        return 'obtusangulo'
    if a ** 2 < b ** 2 + c ** 2:
        return 'acutangulo'
    else:
        return '?'


class Triang:
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c
        self.valido = istriang(a, b, c)
        self.classificacao_lado = classificar_lado(a, b, c)
        self.classificacao_angulo = classificar_angulo(a, b, c)


A, B, C = [float(item) for item in input().split()]  # 5 5 5
triangulo = Triang(A, B, C)
print(triangulo.classificacao_lado)
print(triangulo.classificacao_angulo)

Also your function classificar_angulo does not handle all possible inputs. It needs an else statement or to return some default value if none of the if conditions above are met. I have just changed it to return '?'. You may have a better idea about what it should return.