What happens when I import a module twice in python

Question

I have doubts that I want to be cleaned.

Consider the following module named ex_1.py :

 print("Hello, I'm ex_1") def greet(name): print("Hello, "+name+" nice to meet you! ") 

Now consider another file called 1_client_ex_1.py that will import the ex_1.py module.

 import ex_1.py 

Now, when I run this file, I get the output as:

 Hello, I'm ex_1 

As expected.

But when I go to 1_client_ex_1.py in:

 import ex_1.py import ex_1.py 

and execute it, it still only prints Hello, I'm ex_1 once. Shouldn't I print twice?

Answer 1

Nothing, if the module is already imported, it does not load again.

You simply get a link to a module that has already been imported (it will be obtained from sys.modules ).

To get a list of already imported modules, you can find sys.modules.keys() (note that urllib imports a lot of other modules here):

 >>> import sys >>> print len(sys.modules.keys()) 44 >>> print sys.modules.keys() ['copy_reg', 'sre_compile', '_sre', 'encodings', 'site', '__builtin__', 'sysconfig', '__main__', 'encodings.encodings', 'abc', 'posixpath', '_weakrefset', 'errno', 'encodings.codecs', 'sre_constants', 're', '_abcoll', 'types', '_codecs', 'encodings.__builtin__', '_warnings', 'genericpath', 'stat', 'zipimport', '_sysconfigdata', 'warnings', 'UserDict', 'encodings.utf_8', 'sys', 'virtualenvwrapper', '_osx_support', 'codecs', 'readline', 'os.path', 'sitecustomize', 'signal', 'traceback', 'linecache', 'posix', 'encodings.aliases', 'exceptions', 'sre_parse', 'os', '_weakref'] >>> import urllib >>> print len(sys.modules.keys()) 70 >>> print sys.modules.keys() ['cStringIO', 'heapq', 'base64', 'copy_reg', 'sre_compile', '_collections', '_sre', 'functools', 'encodings', 'site', '__builtin__', 'sysconfig', 'thread', '_ssl', '__main__', 'operator', 'encodings.encodings', '_heapq', 'abc', 'posixpath', '_weakrefset', 'errno', '_socket', 'binascii', 'encodings.codecs', 'urllib', 'sre_constants', 're', '_abcoll', 'collections', 'types', '_codecs', 'encodings.__builtin__', '_struct', '_warnings', '_scproxy', 'genericpath', 'stat', 'zipimport', '_sysconfigdata', 'string', 'warnings', 'UserDict', 'struct', 'encodings.utf_8', 'textwrap', 'sys', 'ssl', 'virtualenvwrapper', '_osx_support', 'codecs', 'readline', 'os.path', 'strop', '_functools', 'sitecustomize', 'socket', 'keyword', 'signal', 'traceback', 'urlparse', 'linecache', 'itertools', 'posix', 'encodings.aliases', 'time', 'exceptions', 'sre_parse', 'os', '_weakref'] >>> import urllib #again! >>> print len(sys.modules.keys()) #has not loaded any additional modules 70 

Give him a whirlwind:

 import sys >>> sys.modules["foo"] = "bar" # Let pretend we imported a module named "foo", which is a string. >>> print __import__("foo") bar # Not a module, that my string! 

As you can see, if the module is not found by sys.modules , you just get a new link to it. What is it.

---

Please note that this means that the modules must be designed so as not to have any side effects (such as printed materials) when they are imported .

Rebooting modules outside an interactive session is usually not a very good practice (although it does have its own use cases). Other answers detail how you do it.

Answer 2

Any python script will reload the module only once. To reload it, use:

 reload() # Python 2 

and

 imp.reload() # Python 3 

Answer 3

A module is loaded only the first time the import statement is executed. See Also reload() and this question . You can examine sys.modules to find out which modules are already loaded.