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1. Opposite person: as a space placeholder

I see it as a concise form of annotation, which is tantamount to saying, "reserve a place here, and then fill in the specific code implementation later."

For example, in a multi-tier if-elif-else structure, we can write the judgment condition first, and then write pass in the corresponding block, and then slowly improve it later.

For example, in the example given above, we can first write the class / function name and its input parameters, then pass the body code, and then slowly populate it later.

Pass is easy to write, and because it is a keyword, IDE gives a conspicuous color distinction, so it is more convenient than we write comments.

As a space placeholder, pass is mainly convenient for us to conceive the local code structure, and has a certain auxiliary reminder effect.

However, as a form of annotation, it is too thin to compare with "# todo: xxxx", which is also highlighted by IDE and has a clearer meaning. Although easy to write, it also introduces a seemingly superfluous keyword pass.

So, from a spatial placeholder perspective, pass is not a necessary design element in a programming language.

With it, we can express the semantics of "there's something here, but skip it temporarily", but without it, we can replace it with annotated content.

2. For machines: for grammatical integrity

For the use of the previous article, the location where pass appears in the code is theoretically unlimited.

However, when we use pass most often, it is basically on the next line of the colon, and this is the only statement in the indented block of code at that layer.

We can imagine what would happen if we didn't write it.

The answer is to report an indentation error: IndentationError: expected an indented block

# remove the pass of the function body, and an error will be reported

Def func ():

Func ()

Because Python uses indentation to divide code blocks, and colons indicate that new indented code blocks are about to appear, this example reports a lack of indented code blocks.

What if we replace it with the notes mentioned earlier?

# replace the pass of the function body with comments

Def func ():

# todo: there is something here that you can make up later.

Func ()

If you write it this way, you will get an error: IndentationError: expected an indented block

The reason is that the comment is not valid grammatical content, it will be ignored by the Python interpreter (ignore), unlike the pass statement is "valid grammatical content, but skip".

That is, the indented code block must contain syntactically meaningful content, and the following examples are valid:

Def func ():

"" this is a string ""

Def func2 ():

123456

When defining a function, Python must include the function body, that is, it contains both declaration and definition semantics, unlike some languages that can only use declaration semantics, that is, void test ();.

However, because Python does not use curly braces, it cannot define empty functions directly, as some languages do, that is, void test () {}.

Based on the above analysis, when Python defines an empty function, it must have a legal function body, so a pass statement is designed to represent the null operation. It is to complement the integrity of syntax, together with colons, equivalent to a pair of empty curly braces in other languages.

From the perspective of grammatical integrity, it is a necessary design element, and if not, it must be replaced by similar empty statements or special symbols.

On the human side, pass can mean "skip temporarily" and, as a temporary placeholder, will eventually be replaced by the actual code implementation; on the machine side, it can mean "skip directly", just to complement the syntax logic and will not be replaced by other code.

Other languages do not have a special statement or symbol to represent such placeholders (that is, semantic deficiencies), but they do not need to bother to design a keyword to make up for grammatical integrity (that is, grammatical completeness).

Back to the question at the beginning of this article: why does Python have a pass statement, what problems (benefits) can it solve, and what problems (disadvantages) will it cause without it?

Python uses pass statements to support blocks of code for purely empty operations (empty functions, empty classes, empty loop control blocks, and so on), with which an additional placeholder semantics can be expressed.

The former is for machines, must have, equivalent to the role of empty curly braces in other languages; the latter is for people, non-essential, can be expressed in annotations, but because Python designed this statement, this usage is sometimes quite convenient.

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