لطفا منتظر باشید ...
Recipe 2.21. Dynamically Changing the PythonSearch Path
Credit: Robin Parmar
Problem
Modules must be on the Python search path
before they can be imported, but you don't want to
set a huge permanent path because that slows performanceso,
you want to change the path
dynamically.
Solution
We simply conditionally add a
"directory" to
Python's sys.path, carefully
checking to avoid duplication:
def AddSysPath(new_path):
"" AddSysPath(new_path): adds a "directory" to Python's sys.path
Does not add the directory if it does not exist or if it's already on
sys.path. Returns 1 if OK, -1 if new_path does not exist, 0 if it was
already on sys.path.
""
import sys, os
# Avoid adding nonexistent paths
if not os.path.exists(new_path): return -1
# Standardize the path. Windows is case-insensitive, so lowercase
# for definiteness if we are on Windows.
new_path = os.path.abspath(new_path)
if sys.platform == 'win32':
new_path = new_path.lower( )
# Check against all currently available paths
for x in sys.path:
x = os.path.abspath(x)
if sys.platform == 'win32':
x = x.lower( )
if new_path in (x, x + os.sep):
return 0
sys.path.append(new_path)
# if you want the new_path to take precedence over existing
# directories already in sys.path, instead of appending, use:
# sys.path.insert(0, new_path)
return 1
if _ _name_ _ == '_ _main_ _':
# Test and show usage
import sys
print 'Before:'
for x in sys.path: print x
if sys.platform == 'win32':
print AddSysPath('c:\\Temp')
print AddSysPath('c:\\temp')
else:
print AddSysPath('/usr/lib/my_modules')
print 'After:'
for x in sys.path: print x
Discussion
Modules must be in directories that are on the Python search path
before they can be imported, but we don't want to
have a huge permanent path because doing so slows down every import
performed by every Python script and application. This simple recipe
dynamically adds a "directory" to
the path, but only if that directory exists and was not already on
sys.path.sys.path is a list, so it's easy
to add directories to its end, using
sys.path.append. Every import
performed after such an append will automatically
look in the newly added directory if it cannot be satisfied from
earlier ones. As indicated in the Solution, you can alternatively use
sys.path.insert(0, . . . so that the newly added
directory is searched before ones that were
already in sys.path.It's no big deal if sys.path ends
up with some duplicates or if a nonexistent directory is accidentally
appended to it; Python's import
statement is clever enough to shield itself against such issues.
However, each time such a problem occurs at import time (e.g., from
duplicate unsuccessful searches, errors from the operating system
that need to be handled gracefully, etc.), a small price is paid in
terms of performance. To avoid uselessly paying such a price, this
recipe does a conditional addition to sys.path,
never appending any directory that doesn't exist or
is already in sys.path. Directories appended by
this recipe stay in sys.path only for the duration
of this program's run, just like any other dynamic
alteration you might do to sys.path.
See Also
Documentation for the sys and
os.path modules in the Library
Reference and Python in a
Nutshell.
