Python function parameter

In the previous article we learned about the built-in Python function and the user-defined Python function with customizable number of parameters. You will know how to define a function using the default parameters, keyword and custom parameters in this article.

Depending on how you define the Python function in any way we can call different functions, of course, no matter how you call the function, the final result is still reasonable and the same.

Parameters of Python functions

In the user-defined Python function, we already know how to define and call a function, like the example below:

 def Xin_chao(ten,loi_chao): 
  """Hàm Xin_chao chào một người 
 với thông điệp cho trước """ 
  print("Xin chào",ten + ', ' + loi_chao) 
 
 Xin_chao("Nam","đọc bài trên TipsMake.com vui vẻ nha!") 

When running the above code, we will get the text on the screen as follows:

 Xin chào Nam, đọc bài trên TipsMake.com vui vẻ nha! 

As you can see, this Xin_chao() function has 2 parameters. Therefore, if we call the function with two parameters, it will run "well" and will not encounter any error messages.

If the function is called with a different parameter number 2, the interpreter will display an error message. If you want, you can try calling the Xin_chao() function with one parameter and no parameters.

 # Error message when calling Xin_chao () function with 1 parameter 
 TypeError: Xin_chao () missing 1 argument positional required: 'loi_chao' 

 # Error message when calling Xin_chao () function without parameters 
 TypeError: Xin_chao () missing 2 required positional arguments: 'ten' and 'loi_chao' 

Besides defining functions with a fixed number of parameters, you can use the methods we introduce below to create custom parameters for the function:

Default parameter in Python function

In Python, the parameter of the function can have a default value. We can provide this default value for a parameter by using the assignment operator ( = ) as the example below:

 def Xin_chao(ten, loi_chao = "đọc bài TipsMake.com vui nha!"): 
  """ 
  Hàm này chào một người với thông điệp cho trước. 
 
  Nếu thông điệp không được cung cấp, 
  nó sẽ được mặc định là "đọc bài TipsMake.com vui nha!" 
  """ 
 print("Xin chào",ten + ', ' + loi_chao) 
 
 Xin_chao("Hải") 
 Xin_chao("Dũng","bạn khỏe không?") 

When running the above code, we have the following output:

 Xin chào Hải, đọc bài TipsMake.com vui nha! 
 Xin chào Dũng, bạn khỏe không? 

In this function, the ten parameter has no default value and is required when calling the function. The parameter loi_chao has a default value of "đọc bài TipsMake.com vui nha!" . Therefore, when calling the function you can provide this parameter or not. If the parameter loi_chao , it will override the default value.

In a function, the number of parameters with default values ​​is not limited, but once there are default parameters, all parameters on the right of the default parameter must also have default values. That is, non-default parameters cannot be behind default parameters. For example, if you define the Xin_chao () function as follows:

def Xin_chao(loi_chao = "đọc bài TipsMake.com vui nha!",ten):

Then will receive an error message:

 SyntaxError: non-default argument follows default argument 

The keyword parameter in Python

When calling a function with several values, these values ​​will be assigned to the parameters according to their location. For example, in the Xin_chao () function above, when calling Xin_chao("Dũng","bạn khỏe không?") , The "Dũng" value is assigned to the ten parameter and the value "bạn khỏe không?" assigned to loi_chao .

Python allows calling functions by keyword parameters. When calling a function this way, the order (position) of the parameter may change, but the result is still valid and similar to other function calls.

 # 2 tham số keyword 
 Xin_chao(ten = "Dũng",loi_chao = "bạn khỏe không?") 
 # 2 tham số keyword, thứ tự tham số thay đổi 
 Xin_chao(loi_chao = "bạn khỏe không?",ten = "Dũng") 
 # 1 tham số keyword, 1 theo vị trí 
 Xin_chao("Dũng",loi_chao = "bạn khỏe không?") 
 

As you can see above, it is possible to combine parameters by keyword or location when calling a function. But remember that the parameters keyword must go after the parameter by location. If the parameter according to the position after the keyword parameter you will receive an error message:

Xin_chao(ten = "Dũng","bạn khỏe không?")

Error message:

 SyntaxError: non-keyword argument after keyword argument 

Custom parameters in Python functions

Sometimes, we do not know in advance the number of parameters that will be passed to the function, it is time to use the function call with the number of custom parameters.

In the function definition, we use an asterisk * before the parameter name to denote the parameter type, as the example below:

 def Xin_chao(*ten_chao): 
  """ 
  Hàm này sẽ chào một danh sách người cho trước 
  """ 
  for ten in ten_chao: 
  print("Xin chào",ten) 
 
 Xin_chao("Hải","Hoa","Công","Sơn") 

When running the above code we get the following result:

 Xin chào Hải 
 Xin chào Hoa 
 Xin chào Công 
 Xin chào Sơn 

Here, we call the function with many parameters. This parameter is "wrapped" into a tuple before being passed into the function. Inside the function, we use the for loop to get all the parameters in tuple.

In the next lesson, we will learn about Python recursion.

Next lesson: Recursive function in Python

Last lesson: Python function is user defined

Don't forget to do your Python homework.