Should import statements always be at the top of a module?

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Module importing is quite fast, but not instant. This means that:

  • Putting the imports at the top of the module is fine, because it’s a trivial cost that’s only paid once.
  • Putting the imports within a function will cause calls to that function to take longer.

So if you care about efficiency, put the imports at the top. Only move them into a function if your profiling shows that would help (you did profile to see where best to improve performance, right??)

The best reasons I’ve seen to perform lazy imports are:

  • Optional library support. If your code has multiple paths that use different libraries, don’t break if an optional library is not installed.
  • In the __init__.py of a plugin, which might be imported but not actually used. Examples are Bazaar plugins, which use bzrlib‘s lazy-loading framework.

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