Importing Modules by Name String

The module name in an import or from statement is a hardcoded variable name. Sometimes, though, your program will get the name of a module to be imported as a string at runtime (e.g., if a user selects a module name from within a GUI). Unfortunately, you can’t use import statements directly to load a module given its name as a string—Python expects a variable name, not a string. For instance:

>>> import "string"
  File "<stdin>", line 1
    import "string"
                  ^
SyntaxError: invalid syntax

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

It also won’t work to simply assign the string to a variable name:

x = "string"
import x

Here, Python will try to import a file x.py, not the string module—the name in an import statement both becomes a variable assigned to the loaded module and identifies the external file literally.

To get around this, you need to use special tools to load a module dynamically from a string that is generated at runtime. The most general approach is to construct an import statement as a string of Python code and pass it to the exec built-in function to run (exec is a statement in Python 2.6, but it can be used exactly as shown here—the parentheses are simply ignored):

>>> modname = "string"
>>> exec("import " + modname)      # Run a string of code
>>> string                         # Imported in this namespace
<module 'string' from 'c:\Python30\lib\string.py'>

The exec function (and its cousin for expressions, eval) compiles a string of code and passes it to the Python interpreter to be executed. In Python, the byte code compiler is available at runtime, so you can write programs that construct and run other programs like this. By default, exec runs the code in the current scope, but you can get more specific by passing in optional namespace dictionaries.

The only real drawback to exec is that it must compile the import statement each time it runs; if it runs many times, your code may run quicker if it uses the built-in __import__ function to load from a name string instead. The effect is similar, but __import__ returns the module object, so assign it to a name here to keep it:

>>> modname = "string"
>>> string = __import__(modname)
>>> string
<module 'string' from 'c:\Python30\lib\string.py'>