{"id":39214,"date":"2025-11-08T09:08:38","date_gmt":"2025-11-08T17:08:38","guid":{"rendered":"https:\/\/cloudinary.com\/blog\/?p=39214"},"modified":"2025-11-08T09:08:39","modified_gmt":"2025-11-08T17:08:39","slug":"how-to-skip-an-optional-function-argument-using-a-ternary-operator-in-python","status":"publish","type":"post","link":"https:\/\/cloudinary.com\/blog\/questions\/how-to-skip-an-optional-function-argument-using-a-ternary-operator-in-python\/","title":{"rendered":"How to skip an optional function argument using a ternary operator in Python?"},"content":{"rendered":"\n

You have a function with optional parameters and you want to pass one of them only when a condition is true. In many languages, it feels natural to try an inline conditional and move on. In Python, the details matter: passing None<\/code> is not the same as skipping an argument entirely, and there is no syntax to \u201comit\u201d an argument from inside a single expression.<\/p>\n\n\n\n

Question:<\/h2>\n\n\n\n

Hi all,<\/em>
I have a function with optional keyword arguments, and I only want to pass one when a condition holds. I tried using a ternary at the call site but Python still expects a value. What is the correct pattern?<\/em>

Concretely: How to skip an optional function argument using a ternary operator in Python? I want something concise that reads nicely in code reviews.<\/em><\/p>\n\n\n\n

Answer:<\/h2>\n\n\n\n

Short answer: you cannot omit an argument from inside a single argument expression. Python evaluates the argument list before the function is called, so an inline expression must still produce a value. Long answer? Let\u2019s break it down.<\/p>\n\n\n\n

1) Use a conditional expression across two separate calls<\/h3>\n\n\n
def f(a, b=10<\/span>):\n\u00a0 \u00a0 return<\/span> a + b\n\ncond = True<\/span>\nresult = f(5<\/span>, b=20<\/span>) if<\/span> cond else<\/span> f(5<\/span>)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
This keeps call sites explicit and works well for one or two options. It is also the most readable in many codebases.<\/p>\n\n\n\n
2) Use kwargs unpacking to include a key only when needed<\/h3>\n\n\n\n
Build a small dictionary conditionally and unpack it with **<\/code>. This lets you stay on one line while truly skipping the argument.<\/p>\n\n\n
def f(a, b=10<\/span>, c=0<\/span>):\n\u00a0 \u00a0 return<\/span> a + b + c\n\ncond_b = True<\/span>\ncond_c = False<\/span>\nresult = f(5<\/span>, **({'b'<\/span>: 20<\/span>} if<\/span> cond_b else<\/span> {}), **({'c'<\/span>: 7<\/span>} if<\/span> cond_c else<\/span> {}))<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
3) For multiple optionals, assemble a kwargs dict first<\/h3>\n\n\n
def f(a, b=10<\/span>, c=0<\/span>, d=None):\n\u00a0 \u00a0 ...\n\nkwargs = {}\nif<\/span> use_b:\n\u00a0 \u00a0 kwargs['b'<\/span>] = 20<\/span>\nif<\/span> limit is not None:\n\u00a0 \u00a0 kwargs['c'<\/span>] = limit\nif<\/span> label:\n\u00a0 \u00a0 kwargs['d'<\/span>] = label\n\nresult = f(5<\/span>, **kwargs)<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
This scales better and stays clean when several parameters are conditional.<\/p>\n\n\n\n
4) If you control the function, use a sentinel default<\/h3>\n\n\n\n
Passing None<\/code> is not the same as skipping. If you need to know whether the caller supplied a value, use a private sentinel.<\/p>\n\n\n
SENTINEL = object()\n\ndef f(a, b=SENTINEL):\n\u00a0 \u00a0 if b is SENTINEL:\n\u00a0 \u00a0 \u00a0 \u00a0 b = compute_default()\n\u00a0 \u00a0 ...<\/code><\/span><\/pre>\n\n\n
Now callers can pass b=None as a legitimate value without being conflated with the default case.<\/p>\n\n\n\n
Common pitfalls<\/h3>\n\n\n\n