{"id":12984,"date":"2022-10-02T15:58:16","date_gmt":"2022-10-02T10:28:16","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-read-and-write-a-variable-in-module-a-from-module-b\/"},"modified":"2022-10-02T15:58:16","modified_gmt":"2022-10-02T10:28:16","slug":"solved-read-and-write-a-variable-in-module-a-from-module-b","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-read-and-write-a-variable-in-module-a-from-module-b\/","title":{"rendered":"[Solved] Read and write a variable in module A from module B"},"content":{"rendered":"

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When you execute your moduleA<\/code> you’re running it as a script – essentially a module with the name of __main__<\/code>, not as a ‘normal’ module, and that’s how it gets loaded. If you go and look through sys.modules<\/code> as soon as you start it (before you import moduleB<\/code>) you ain’t gonna find your moduleA<\/code> there but you’ll find module __main__<\/code> (which will soon enough hold a variable<\/code> with value 10<\/code>).<\/p>\n

Then when you import your moduleB<\/code>, it imports moduleA<\/code> – Python tries to find the loaded moduleA<\/code> but there isn’t any, so it tries to load it from the disk and voila! it gets loaded, but this time as moduleA<\/code>. Python then looks for moduleB<\/code> that is imported from moduleA<\/code>, and since it’s already there it doesn’t make any fuss about it despite the cyclic dependency (and if you have something like that in your code – you’re doing it wrong).<\/p>\n

Anyway, since it’s now loaded as moduleA<\/code>, its if __name__ == \"__main__\":<\/code> block evaluates to false so it doesn’t cause any additional fuss. moduleB<\/code> proceeds with its execution, but it also doesn’t match its if __name__ == \"__main__\":<\/code> block so it just gets loaded and sits there.<\/p>\n

Now we’re back in our __main__<\/code> representation of moduleA<\/code> – its if __name__ == \"__main__\":<\/code> block evaluates to true, so it calls its main()<\/code> function, which in turn calls moduleB<\/code>‘s main()<\/code> function, which then changes the variable<\/code> value but in moduleA<\/code>, not in __main__<\/code>. So, now when it gets back to __main__<\/code> and it tries to read its own variable<\/code> value it gets the unchanged version of 10<\/code>. But if you were to print out: print(getattr(sys.modules[\"moduleA\"], \"variable\"))<\/code> you’d see that your moduleB<\/code> indeed changed the variable of moduleA<\/code>.<\/p>\n

If you really want to force the change in main<\/code>, try having your moduleB<\/code> like:<\/p>\n

import sys\n\ndef main():\n    moduleA = sys.modules[\"__main__\"]  # this is the executing `moduleA` in your case\n    print(\"From moduleB, variable =\", moduleA.variable, \" before change.\")\n    moduleA.variable = 20\n    print(\"From moduleB, variable =\", moduleA.variable, \" after change.\")\n\nif __name__ == \"__main__\":\n    main()\n<\/code><\/pre>\n
Which will, running your test case from moduleA<\/code>, print out:<\/p>\n
('From moduleA, variable=\", 10, \" before change.')\n('From moduleB, variable=\", 10, \" before change.')\n('From moduleB, variable=\", 20, \" after change.')\n('From moduleA, variable=\", 20, \" after change.')\n<\/code><\/pre>\n
That’s what’s going on here, and the same is happening with your static vars example – you’re consistently targeting the ‘wrong’ module form your moduleB<\/code>. Either way, please do not do this in any sort of a production code or any situation where other developers might end up having to mess with your code – cyclic redundancies are the bane of all things nice.<\/p>\n<\/p><\/div>\n
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solved Read and write a variable in module A from module B <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] When you execute your moduleA you’re running it as a script – essentially a module with the name of __main__, not as a ‘normal’ module, and that’s how it gets loaded. If you go and look through sys.modules as soon as you start it (before you import moduleB) you ain’t gonna find your moduleA … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[349],"class_list":["post-12984","post","type-post","status-publish","format-standard","hentry","category-solved","tag-python"],"yoast_head":"\n[Solved] Read and write a variable in module A from module B - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-read-and-write-a-variable-in-module-a-from-module-b\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Read and write a variable in module A from module B - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] When you execute your moduleA you’re running it as a script – essentially a module with the name of __main__, not as a ‘normal’ module, and that’s how it gets loaded. 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