Prasad, Ramit wrote:
In any event, you shouldn't be worrying about something like overhead until
after your base prorgram is written.
Base programs are written. I was just looking into insight about the mechanics
behind importing :)
For instance, do these libraries normally lazy load? If you have mod1.mod2 and
mod1.mod3 and import mod1 does it usually also internally load mod2/3? import
mod1.mod2 (or do from mod1 import mod2) does it avoid any loading of mod3 or
does it do that as part of loading mod1?
That depends on the module.
In the case of os, it has code like the following:
if 'posix' in _names:
name = 'posix'
linesep = '\n'
from posix import *
try:
from posix import _exit
except ImportError:
pass
import posixpath as path
import posix
__all__.extend(_get_exports_list(posix))
del posix
elif 'nt' in _names:
# ... similar code
elif 'os2' in _names:
# ...
elif 'mac' in _names:
# ...
elif 'ce' in _names:
# ...
elif 'riscos' in _names:
# ...
else:
# raise exception
So in the case of os, no, it does not lazily load path only when needed,
and os.path is available as soon as you import os, without any further
imports.
However, if the module is a more modern package, the situation *may* be
different. A package uses a directory with sub-modules, plus an
__init__.py file. In this case, the behaviour is entirely up to the author.
Here's an example from one of my own Python 3 libraries: I have a
package that (currently) looks something like this:
stats/
+-- __init__.py
+-- co.py
+-- order.py
+-- utils.py
+-- _tests/
+-- test_co.py
... etc.
(greatly simplified).
When you call "import stats", Python reads the file stats/__init__.py,
compiles it and creates a module from it. __init__.py in turn includes a
line "from . import utils". (This is Python 3 syntax, so it doesn't
work with Python 2.) That line imports the stats/utils.py submodule and
makes it available from within stats as stats.utils.
However the sub-modules co.py and order.py are *not* imported by the
main module. They are only loaded on demand, when you say "import
stats.co" or "from stats.order import median" or similar.
So it depends on the module.
--
Steven
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