How to concatenate a string returned by a lambda with other strings?

Question

I'm trying to use a lambda to check if an object (self)'s attribute problematic_object is None. If it is, it should return a "" (an empty string), and if not, str(self.problematic_object). I've tried doing it in 3 ways:

1:

def __str__(self):
    return "string1" + str(self.object) + "\n" + "string2" + self.another_object + "\n" + "string3" + exec(lambda self: str(self.problematic_object) if self.problematic_object!=None else "")

2:

def __str__(self):
    return "string1" + str(self.object) + "\n" + "string2" + self.another_object + "\n" + "string3" + (lambda self: str(self.problematic_object) if self.problematic_object!=None else "")

3:

def __str__(self):
    return "string1" + str(self.object) + "\n" + "string2" + self.another_object + "\n" + "string3" + lambda self: str(self.problematic_object) if self.problematic_object!=None else ""

I get this error in all the cases:

SyntaxError: invalid syntax

I know that this can be done using a normal if-else, but is there any way I can do it using a lambda? This is my first time using a lambda and this kind of an if-else. What is the mistake I'm making? Can a lambda be used like this?

Show a stack trace in all cases. What is actually causing the error? Check for unclosed parens on the previous line btw — Mad Physicist, Mar 25 '22 at 03:34
Can you add the lambda you are trying to use to your question please? — Emrah Diril, Mar 25 '22 at 03:56
There's no reason I can see for using an anonymous function here. Use a ternary expression. — ddejohn, Mar 25 '22 at 05:27
`lambda` creates an anonymous function object -- it needs to be called in order to return anything. This is not a situation where you should use a lambda, period. — ddejohn, Mar 25 '22 at 05:29
I guess this is related too and is a way other than using f-strings: https://stackoverflow.com/a/19213688/16136190. — The Amateur Coder, Jul 16 '22 at 13:45

score 0 · Accepted Answer · answered Mar 25 '22 at 05:34

If self.problematic_object is possibly None, and in that case you just want an empty string, simply use f-strings to add it to your overall string. No need for any boolean logic:

def __str__(self):
    return f"string1{self.object}\nstring2{self.another_object}\nstring3{self.problematic_object}"

If self.problematic_object is None then nothing will be added to the end of the string. If it's not None then its value will be added.

How to concatenate a string returned by a lambda with other strings?

1 Answers1