1.4 learn more about functions

Let's continue with the simplest one last time Python On the basis of the program , Let's take a look at the usage of functions .

We changed the code , Can you understand what this code means ？

str = input()            # Such as input bye,str by ‘bye’
str = str.upper()        # here str by ‘BYE’
print(str)                # Output ‘BYE’

It means the same thing , Just in the middle step, first convert the characters to uppercase , And replaced the original str Variable . You can also see from it , So-called Python Programming is like this , Write line by line , One function per line , It's like piling up wood , Put together a complete function . Of course , Although the function of this code is consistent with the previous , But there are some differences . This is the old code ：

str = input()			# Such as input bye,str by ‘bye’
print(str.upper())		# Output ‘BYE’,str by ‘bye’

In the previous code ,str Always input in lowercase bye, But in this new code , because str Has been replaced , Therefore, the original input content will no longer be available .

You may also notice , There is a text beginning with a pound sign after each code sentence , This is called annotation , Generally, it is some textual annotation and description , For people to understand the meaning of the code .

meanwhile , Even paste this code directly into PyCharm in , You'll find out , The statement content of these comments does not affect the operation of the program , And it is displayed in light gray .

This function is very useful , Sometimes we want to temporarily cancel the execution of a piece of code , Deleting it directly is not a good idea , It's not easy to find the code again , therefore , It can be handled by temporarily commenting into code . such as , We want to temporarily mask the second line of code of this uppercase conversion function , You can click on this line , Press down Ctrl+/, You will find that it automatically becomes a comment ：

Press again and it will change back , This is a toggle key . If you select multiple lines , You can comment on multiple lines .

Maybe some readers will think whether it is possible to write like this ：

print(input())

The effect is completely OK , It takes the input directly as print The parameters of the function , That is, direct output .

thus , We have written three versions of the same function ：

Then why do we define variables str To save the input ？ There is a key problem involved here ： When do I need to define variables ？

The answer is simple , If you need to continue to use the previous data content later , Then you should consider saving it to a specific variable . If direct output is adopted ：

print(input())

The input here is not saved , Although it can be output , But if the input content needs to be processed again later , You will find that you will not be able to get the previous input . conversely , If you have saved the input to str variable ：

str = input()			
print(str.upper())

Then you can pass at any time str Variables get the input again . It's like using money , If you need money tomorrow , Then you need to store the required amount in advance . It's the same thing ！

The problems that beginners often encounter here are the random addition of indentation and unnecessary spaces in the code , It is highly recommended that you do not add unnecessary spaces at will , It can even lead to errors . For example, unnecessary spaces have been added at the beginning of the second and third lines , Led to a mistake （ The meaning of the error message is unexpected code indentation , There are spaces at the beginning of the line. Of course, this line of code is indented ）：

The specific reasons will be explained later . Here you can add a very useful function , Often after writing too much code , There may be many careless spaces , The format will be messy . It is suggested that you often press Ctrl+Alt+L, You'll find out PyCharm Auto layout code , Remove unnecessary spaces , Align code, etc , Very convenient . For example, the above code will be immediately typeset as ：

Only this and nothing more , We give you a brief explanation of the basic Python Program , You can find that it seems to be composed of various variables and functions . From the cognitive level, the meaning of variables and functions , We can make a conclusion ,Python The code of is composed of various variables and functions ：

The most obvious surface difference between variables and functions is that there are parentheses after functions , Even if there is no content . In terms of function , The difference between the two can be understood as , Variables express some static characteristics , Functions express some dynamic functions , Therefore, the combination of the two can completely express the things in the objective world and their related relationships , So as to reproduce these things and relationships through code logic , It realizes the specific logic functions we need （ A kind of thinking similar to programming philosophy ）.

among , Functions seem more strange , Variables can be defined by themselves , But functions often need ready-made functions . occasionally , We're going to have a problem , We don't know which functions ？ This is about to learn , Meet one, know one , in the course of time , You can learn some of the most common functions . Most of the data analysis functions of our course are provided by these functions , We don't need to know the details , Just know how to use them , It is enough to complete the data analysis tasks we need .

Supporting learning resources 、 MOOC video ：

https://www.njcie.com/python/