Ignoring whether it can run successfully, is this expression grammatically valid Python? Try to analyze it from Python's grammar before testing it in an interpreter:
...._(()())._(...())()
A bonus would be explaining how Python parses each part and which failures are syntax errors versus runtime errors.
3 Answers
Yes, it is valid Python syntax, although it is very unlikely to succeed at runtime. The first three dots form the `Ellipsis` literal, and the fourth dot starts an attribute access, so `...._` means `... ._`. The following parentheses are a function call whose argument is an empty tuple expression being called. Then another `._(...)` access is performed, with `...()` as its argument, and the final `()` calls the result. The grammar permits these calls and attribute accesses even though the objects involved normally are not callable or do not have the requested attribute.
The important distinction is syntax versus semantics. Python's grammar allows an arbitrary expression before a call, so an empty tuple or the ellipsis object can appear in those positions. Whether the object is callable, or whether `Ellipsis` has an attribute named `_`, is checked only when the expression runs. In ordinary Python, evaluation therefore raises an exception, but the source itself is grammatically valid.
The expression can be viewed roughly as a chain like this:
`a = Ellipsis._((())())`
`b = a._(Ellipsis())`
`c = b()`
That is only a structural interpretation—the expression will fail during evaluation. Constructs such as `()()` and `...()` are not rejected by the grammar; Python parses them as calls and reports their problems only at runtime.
That conclusion is about normal runtime behavior, not syntax. A different implementation could theoretically make the objects mutable or callable and get farther, while the grammar would remain unchanged.