CODEWITHSUNDEEP
pythonpython-3.x
jiligulu
jiligulu

Reputation: 197

Why is "del" a statement and not a build-in function in Python?

In Python3, del is a statement to be used like del x. Why is del not designed as a built-in function used like del(x)?

Upvotes: 5

Views: 240

Answers (1)

MisterMiyagi
MisterMiyagi

Reputation: 50076

TLDR: This is a result of use-mention distinction. A function cannot operate on the symbol x.

The important distinction between statements and functions is that statements can work on symbols whereas functions can only work on values.

Consider a hypothetical delete function draft:

>>> def delete(*targets):
...     print("Deleting", targets, "[TODO: implement actual deletion]")
...
>>> x = "the value of x"
>>> delete(x)
Deleting ('the value of x',) [TODO: implement actual deletion]

By using x (or an attribute x.y, subscript x[y] or slice x[y:z]) in a function call, it is automatically evaluated to its value. The function only receives the value, not the symbol or even the symbol-in-its-scope – the function thus cannot delete the symbol from its scope, since it knows neither.

Python has no concept of quoting expressions, so there is no way to express "the symbol x". It is possible to use strings to contain symbol names, such as using delattr(x, 'y') instead of del x.y, but not symbol context – a hypothetical delattr('x.y') cannot work, since it does not know the initial x's scope.

Only things built into the grammar can refer to symbols and their context. This means that del must be at least a built-in statement or expression. In parallel to the assignment statement, which operates on the same targets, del is a statement.

Upvotes: 8

Related Questions