About Python Medium pass sentence , It seems very simple （ Only 4 Letters ）, Even beginners without any programming experience can quickly master its usage .
Official documents It's very simple
Briefly ,pass It's a null operation （null operation）, When the interpreter executes it , In addition to checking whether the grammar is legal , Just skip without doing anything .
It goes with return、break、continue and yield And so on , The biggest difference is that it doesn't change the execution order of the program . It's like a note we wrote , Except for one line of code , It doesn't have any impact on the scope .
however , If you have the foundation of other languages , You may be curious ： Why? Python There is such a unique pass sentence , Other languages don't ？
Python So designed , What's the reason ？
It is to solve the common problems that most programming languages have to face , Or because it has a new discovery of its own , So create a new feature ？
let me put it another way ：Python Why would there be pass sentence , What problems can it solve （ benefits ）, Without it , What problems will it cause （ Disadvantage ）？
Next , This paper will analyze from two dimensions .
1、 in personam ： As a space placeholder
I see it as a concise way of annotating , It means “ Let's reserve a space here , Back to the specific code implementation ”.
For example, in multi-layer if-elif-else In structure , We can write down the judgment conditions first , Then write... In the corresponding block pass, I'll improve it later .
For example, the example given above , We can write the class first / Function name and its input parameters , Then skip （pass） The main body of code , Fill in later .
pass It's easy to write , And because it's a keyword ,IDE Will give you a clear color distinction , So it's more convenient than we write notes .
pass As a space placeholder , It is mainly convenient for us to conceive the local code structure , It has a certain auxiliary reminding function .
however , As a way of annotation , It's too thin , It's not as good as writing “# todo: xxxx”, The latter will also be IDE Highlight with color , And the meaning is clearer . Although it's easy to write , But it also introduces a seemingly superfluous keyword pass.
therefore , From the perspective of spatial placeholders ,pass Not a necessary design element in a programming language .
With it , We can express “ There's something here , But skip for a while ” The semantics of the , But without it , You can use comments instead of .
2、 To the machine ： For grammatical integrity
For the usage of the former ,pass The place in the code is theoretically unlimited .
however , We use... Most often pass when , Basically on the next line of the colon , And in the indented code block of this layer , There's only one sentence .（ See above 3 An example , For convenience , Let's just take an empty function as an example ）
We can imagine , If you don't write it , What will happen ？
The answer is an indentation error ：IndentationError: expected an indented block
# Let the body of a function be pass Remove , Will report a mistake
def func():
func()
because Python Use indentation to divide code blocks , A colon indicates the appearance of a new indented block of code , So this example will report a lack of indented code block .
If we replace it with the comments mentioned above , See what happens ？
# Let the body of a function be pass Change to notes
def func():
# todo: There's something here , Fill in later
func()
Write it like this , You will also report mistakes. ：IndentationError: expected an indented block
The reason is that annotations are not valid grammatical content , It will be Python The interpreter ignores （ignore）, Unlike pass Sentence that is “ Effective grammatical content , But skip ”.
in other words , The indented code block must contain syntactic content , The following examples are all valid ：
def func():
""" This is a string """
def func2():
123456
Python When you define a function , Must contain the body of the function , That is, it contains both declaration and definition semantics , You can't just use declarative semantics like some languages , It's written as void test(); .
however , because Python Don't use curly braces , It can't define empty functions directly like some languages , It's written as void test(){} .
Combined with the above analysis ,Python When you define an empty function , There has to be a legal body of functions , Therefore, we design a method to represent null operation pass sentence . It's to complement the integrity of grammar , With colon , Equivalent to a pair of empty curly braces in other languages .
From the perspective of grammatical integrity , It's a necessary design element , If not , It must also be replaced by similar empty statements or special symbols .
On the human side ,pass Can be said “ Skip for now ” The meaning of , As a temporary place holder , It will eventually be replaced by the actual code implementation ; On the machine side , It can express “ Just skip ”, Just to complement grammar and logic , It won't be replaced by other code .
Other languages don't have a special statement or symbol to represent this kind of place holder （ There is a lack of semantics ）, But they don't need to design a keyword to complement the grammatical integrity （ That is, the grammar is complete ）.
Back to the question at the beginning of this article ：Python Why would there be pass sentence , What problems can it solve （ benefits ）, Without it , What problems will it cause （ Disadvantage ）？
Python Use pass sentence , It's a code block to support pure null operations （ Empty function 、 Empty class 、 Empty loop control blocks and so on ）, With it , It can also express the semantics of a place holder .
The former is for machines , There must be , It is equivalent to the function of curly braces in other languages ; The latter is for people , Not a must , It can be expressed in comments , But because Python Designed this statement , This usage is quite convenient sometimes .
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