Importing modules

Peyman Askari peter_peyman_puk at yahoo.ca
Fri Mar 19 11:47:53 CET 2010


I want to write a function which imports modules the first time, and reloads them afterwards, but I am running into problems with global variables and exec. I will include a full script, but let me elaborate first.

Essentially what you need is

def import_or_reload():
 """assume we want to load or reload sys"""
 if 'sys' in dir():
  reload(sys)
else: 
  import sys

but this runs into the problem that sys is imported within the local scope of the function, so you insert a global statement



def import_or_reload2():

 """Add 'global sys'"""
 global sys

 if 'sys' in dir():

  reload(sys)

else: 

  import sys

'sys' is still not in dir() as dir() pertains to the local scope of the function, but one can get around this by creating a local modules list and adding the imported modules to it





def import_or_reload3():


 """Add 'global modules'"""

 global sys
 global modules



 if 'sys' in modules:


  reload(sys)


else: 


  import sys
  modules.append('sys')

now lets add a parameter to the function signature, so any module name can be passed as an argument and loaded





def import_or_reload4(module_name):



 """Add exec"""
 

exec 'global %s'%module_name

 global modules





 if module_name in modules:



  exec 'reload(%s)'%module_name



else: 



  exec 'import %s'%module_name

  exec 'modules.append(\'%s\')'%module_name

but this doesn't work as global does not cooperate with exec
is there a __reload__('X') function like there is an __import__(‘X’) function? 

Also is there a better way to import modules at run time?

Cheers and here is the test script in case you can't access the attachment

def a():
    global modules
    global sys
    import sys

    modules.append('sys')

def b():
    global modules
    global sys

    reload(sys)

def c(module_name):
    global modules
    exec 'global %s'%module_name
    exec 'import %s'%module_name

    modules.append(module_name)

def test():
    global modules
    global sys

    #create the module list to contain all the modules
    modules=[]

    print 'originally dir() returns:'
    print dir()

    a()
    print 'function a() properly imports the following module:'
    print sys
    print 'is %s in %s->%s'%('sys',modules,'sys' in modules)
    
    b()
    print 'function b() properly reloads the following module:'
    print sys
    print 'is %s still in %s->%s'%('sys',modules,'sys' in modules)

    try:
        c('os')
        print 'function c() properly imports the following module:'
    except:
        print 'function c() failed to import module os'
        print 'is %s in %s->%s'%('os',modules,'os' in modules)

    try:
        print os
        print 'is %s still in %s->%s'%('os',modules,'os' in modules)
    except:
        print 'os was loaded, but is not visible outside of the scope of c()'
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