The reason for the error is NameResolutionError(self.host,self,e)frome, which is an exception type in the urllib3 library. The reason for this error is that DNS resolution failed, that is, the host name or IP address attempted to be resolved cannot be found. This may be caused by the entered URL address being incorrect or the DNS server being temporarily unavailable. How to solve this error There may be several ways to solve this error: Check whether the entered URL address is correct and make sure it is accessible Make sure the DNS server is available, you can try using the "ping" command on the command line to test whether the DNS server is available Try accessing the website using the IP address instead of the hostname if behind a proxy

The difference between null and NULL in C language is: null is a macro definition in C language, usually used to represent a null pointer, which can be used to initialize pointer variables, or to determine whether the pointer is null in a conditional statement; NULL is a macro definition in C language A predefined constant in , usually used to represent a null value, used to represent a null pointer, null pointer array or null structure pointer.

In JavaScript, both undefined and null represent the concept of "nothing": 1. undefined represents an uninitialized variable or a non-existent property. When a variable is declared but no value is assigned to it, the value of the variable is undefined , when accessing properties that do not exist in the object, the returned value is also undefined; 2. null represents an empty object reference. In some cases, the object reference can be set to null to release the memory it occupies.

The difference between null and undefined is: 1. Semantic meaning; 2. Usage scenarios; 3. Comparison with other values; 4. Relationship with global variables; 5. Relationship with function parameters; 6. Nullability check; 7. Performance considerations; 8. Performance in JSON serialization; 9. Relationship with types. Detailed introduction: 1. Semantic meaning, null usually means knowing that this variable will not have any valid object value, while undefined usually means that the variable has not been assigned a value, or the object does not have this attribute; 2. Usage scenarios, etc.

Before introducing the usage of self in Python, let’s first introduce the classes and instances in Python. We know that the most important concepts of object-oriented are classes and instances. Classes are abstract templates, such as abstract things like students. , can be represented by a Student class. Instances are specific "objects" created based on classes. Each object inherits the same methods from the class, but its data may be different. 1. Take the Student class as an example. In Python, the class is defined as follows: classStudent(object):pass(Object) indicates which class the class inherits from. The Object class is all

Usage: 1. Initialize the reference type variable to null, indicating that the variable does not currently point to any object; 2. Set the reference type variable to null, which can release the memory space of the object referenced by the variable and help the garbage collector to recover This object; 3. Use null to check whether a reference is empty. You can avoid the occurrence of NullPointerException by judging whether the reference is null. 4. Use null in conditional judgment to judge whether a reference is empty.

Both null and undefined indicate a lack of value or an undefined state. Depending on the usage scenario, there are some guiding principles for choosing to use null or undefined: 1. When you need to clearly indicate that a variable is empty or invalid, you can use null; 2. When a variable has been declared but not yet assigned a value, it will be set to undefined by default; 3. When you need to check whether a variable is empty or undefined, use the strict equality operator "===" to determine whether the variable is null or undefined. .

How to remove nulls from PHP arrays: 1. Use the "foreach($arr as $k=>$v){...}" method to remove null values ??from the array; 2. Use the while syntax structure to remove null values; 3. Use The array_filter function filters the array and removes null values.