Where does python module data come from?

Where does python's module data come from? The first thing you need to know is how Python finds this package module. Then, if you have a deeper understanding of the concept of namespace, you can know where python's module data comes from.

Search path

When you import a module, the Python parser searches for module locations in the following order:

1. Current directory

2. If it is not in the current directory, Python will search every directory under the shell variable PYTHONPATH.

3. If neither is found, Python will check the default path. Under UNIX, the default path is generally /usr/local/lib/python/.

The module search path is stored in the sys.path variable of the system module. The variables contain the current directory, PYTHONPATH and the default directory determined by the installation process.

Namespace and Scope

A variable is the name (identifier) ​​that holds the matching object. A namespace is a dictionary containing variable names (keys) and their corresponding objects (values).

A Python expression can access variables in the local namespace and global namespace. If a local variable has the same name as a global variable, the local variable overrides the global variable.

Each function has its own namespace. The scoping rules for class methods are the same as for regular functions.

Python will intelligently guess whether a variable is local or global, assuming that any variable assigned within a function is local.

Therefore, if you want to assign a value to a global variable within a function, you must use the global statement.

The expression of global VarName will tell Python that VarName is a global variable, so that Python will not look for this variable in the local namespace.

For example, we define a variable Money in the global namespace. We then assign a value to the variable Money within the function, and then Python will assume that Money is a local variable. However, we did not declare a local variable Money before accessing it, and the result is an UnboundLocalError. This problem can be solved by canceling the comment before the global statement.

#!/usr/bin/python
# -*- coding: UTF-8 -*-
 
Money = 2000
def AddMoney():
   # 想改正代码就取消以下注释:
   # global Money
   Money = Money + 1
 
print Money
AddMoney()
print Money

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