Re: [Visualpython-users] Let's hash out the documentation for restrictive imports

[Guy K. K. <g....@ma...>]

On Sat, 09 Oct 2010 10:44:29 Bruce Sherwood wrote:
> This proposal assumed that the "libvisual" file was in the visual
> folder, in which case I thought that relative imports were now
> favored, for various reasons. As I understand it, the main issue is
> that there could be a module named "foo" at a higher level in the
> search path, in which case "import foo" might not pick up our foo, but
> someone else's foo. Have I misunderstood something?

Well, not really. It's just that the use of relative imports is mainly 
intended for corner cases, but it's discouraged to be used on a general basis. 
Of course, things could break if there's a clash between two "foo" modules. 
This is called "shadowing". One common case is for example when people call a 
variable "file" and therefore are shadowing the built-in "file" type.

In other cases, this behaviour is exactly desired, so for example when people 
*want* to use a specific implementation as a drop-in replacement for an 
otherwise used package. This is for example quite common with the XML parser 
through the ElementTree API. Many APIs are designed to be compatible with each 
other, another one of those are the ones of the threading/processing modules. 
So a user can choose which one to use (for various reasons).

So what it boils down to is to commonly use absolute imports rather than 
relative ones, unless something demands a relative import for specific 
reasons. From the Python documentation [1]: "Relative imports can lead to a 
module being initialized twice, leading to confusing bugs. See PEP 328 [2] for 
details."

You can read up more on this and related issues more on [1], [2] and [3].

Guy


[1] http://docs.python.org/faq/programming.html#what-are-the-best-practices-
for-using-import-in-a-module

[2] http://docs.python.org/release/2.5/whatsnew/pep-328.html

[3] http://bugs.python.org/issue10031

Guy


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Guy K. Kloss
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