Develop Reference

Pros and cons of using exec for importing a specific module?

I would like to find out disadvantages of using exec for imports. One of the files serves as interface towards real implementations of specific functionalities depending on chosen project (framework is intended to work on several projects).
First use-case goes like this:
exec ("from API.%s.specific_API_%s import *" % (project, project))
This way I don't have to hard code anything except the variable project which is injected in the interface-module itself.
This is the other way:
if project == 'project_one':
from API.project_one.specific_API_project_one import *
elif project == 'project_two':
from API.project_two.specific_API_project_two import *
elif project == 'project_three':
from API.project_three.specific_API_project_three import *
This way I have to alter this interface-file each time new project is added to be supported.

If you need programmatic way to import modules, please use importlib or __import__ (for really specific cases). Reasons — don't re-invent the wheel, there's way to do what you want without exec. If your project variable coming from outer world, exec is a huge security issue.
Wildcard imports considered bad practice — it makes harder to maintain your codebase afterwards.
Oversimplified example of issues with exec by executing arbitrary code:
module = 'request'
func = 'urlopen'
exec("from urllib.%s import %s" % (module, func))
func = 'urlopen; print("hello python")'
exec("from urllib.%s import %s" % (module, func))
yes, your example is harder to forge, but problem stays — giving python arbitrary code to execute is overkill (with potential security gap), when you have tool built exactly for your purpose — programatically importing modules.

Best way to store JSON data in a Python Module

I have a Python module I'm developing that's includes a big JSON file as part of the data it relies on. I want Python users to be able to import the JSON file as a Python variable and for users of other programming languages, to be able to use the JSON file directly.
So, what I'm trying to figure out is what's the best way to make the JSON object "importable". Right now, my solution is in __init__.py:
import json
import os
with open(os.path.join(os.path.dirname(os.path.realpath(__file__)), 'seals.json')) as f:
seals_data = json.load(f)
Then Python devs can call:
from my_module import seals_data
And it more or less works, but this feels weird to me, and I want to make sure there isn't a cleaner way to make the json importable.

Your option is probably the best way to store the data as a python structure in a module for someone to use. The only down side I see with it is anything that imports that module will have to wait for it to do IO, which is not ideal, esp if the module is ever used for things that do not involved that data.
I would use a class, or function to lazily load that data, allowing that IO hit to be offset to only when that data is requested.
Here is an example:
SEALS_DATA = None
def get_seals_data():
global SEALS_DATA
if SEALS_DATA is None:
SEALS_DATA = json.load(open("%s/seals.json"%(os.path.dirname(__file__))))
return SEALS_DATA
After they import the module they will have no deal until the first time they call get_seals_data(). Any call after that should already have the data loaded.

Is there an accepted way in Python to import a module from within a function?

I have written a script for XBMC which optionally downloads a dll and then imports a module that depends on that dll if the download was successful.
However, placing the import inside a function generates a Python syntax warning.
Simplified example:
1 def importIfPresent():
2 if chkFunction() is True:
3 from myOptionModule import *
Line 3 generates the warning, but doesn't stop the script. I can't place this code at the start outside of a function because I need to generate dialog boxes to prompt the download and then hash the file once it is downloaded to check success. I also call this same code at startup in order to check if the user has already downloaded the dll.
Is there a different/better way to do this without generating the syntax warning? Or should I just ignore the warning and leave it as is?
Thank you! Using the useful responses below, I now have:
import importlib
myOptionalModule = None
def importIfPresent():
if chkFunction is True:
try:
myOptionalModule = importlib.import_module('modulex')
except ImportError:
myOptionalModule = None
...
importIfPresent()
...
def laterFunction():
if myOptionalModule != None:
myParam = 'something expected'
myClass = getattr(myOptionalModule, 'importClassName')
myFunction = getattr(myClass, 'functionName')
result = myFunction(myClass(), myParam)
else:
callAlternativeMethod()
I am posting this back mainly to share with other beginners like myself the way I learned through the discussion to use the functionality of a module imported this way instead of the standard import statement. I'm sure that there are more elegant ways of doing this that the experts will share as well...

You're not getting the warning for doing an import inside a function, you're getting the warning for using from <module> import * inside a function. Doing a In Python3, this actually becomes a SyntaxError, not a SyntaxWarning. See this answer for why wildcard imports like this in general, and expecially inside functions are discouraged.
Also, this code isn't doing what you think it does. When you do an import inside a function, the import only takes affect inside the function. You're not importing that module into the global namespace of the file, which I believe is what you're really trying to do.
As suggested in another answer importlib can help you here:
try:
import myOptionModule as opt
except ImportError:
opt = None
def importIfPresent():
global opt
if chkFunction() is True:
opt = importlib.import_module("myOptionModule")

I beleive you need to use the importlib library to facilitate this.
The code would be at the top of the mod:
import importlib
then replace "from myOptionModule import *" with "module = importlib.import_module(myOptionModule)". You can then import the defs/classes you want or import them all by using getattr(module,NAME(S)TOIMPORT).
See if that works.
Check out chapter 30 and 31 of Learning Python by Lutz for more info.

Import python module over the internet/multiple protocols or dynamically create module

Is it possible to import a Python module from over the internet using the http(s), ftp, smb or any other protocol? If so, how? If not, why?
I guess it's about making Python use more the one protocol(reading the filesystem) and enabling it to use others as well. Yes I agree it would be many folds slower, but some optimization and larger future bandwidths would certainly balance it out.
E.g.:
import site
site.addsitedir("https://bitbucket.org/zzzeek/sqlalchemy/src/e8167548429b9d4937caaa09740ffe9bdab1ef61/lib")
import sqlalchemy
import sqlalchemy.engine

Another version,
I like this answer. when applied it, i simplified it a bit - similar to the look and feel of javascript includes over HTTP.
This is the result:
import os
import imp
import requests
def import_cdn(uri, name=None):
if not name:
name = os.path.basename(uri).lower().rstrip('.py')
r = requests.get(uri)
r.raise_for_status()
codeobj = compile(r.content, uri, 'exec')
module = imp.new_module(name)
exec (codeobj, module.__dict__)
return module
Usage:
redisdl = import_cdn("https://raw.githubusercontent.com/p/redis-dump-load/master/redisdl.py")
# Regular usage of the dynamic included library
json_text = redisdl.dumps(host='127.0.0.1')
Tip - place the import_cdn function in a common library, this way you could re-use this small function
Bear in mind It will fail when no connectivity to that file over http

In principle, yes, but all of the tools built-in which kinda support this go through the filesystem.
To do this, you're going to have to load the source from wherever, compile it with compile, and exec it with the __dict__ of a new module. See below.
I have left the actually grabbing text from the internet, and parsing uris etc as an exercise for the reader (for beginners: I suggest using requests)
In pep 302 terms, this would be the implementation behind a loader.load_module function (the parameters are different). See that document for details on how to integrate this with the import statement.
import imp
modulesource = 'a=1;b=2' #load from internet or wherever
def makemodule(modulesource,sourcestr='http://some/url/or/whatever',modname=None):
#if loading from the internet, you'd probably want to parse the uri,
# and use the last part as the modulename. It'll come up in tracebacks
# and the like.
if not modname: modname = 'newmodulename'
#must be exec mode
# every module needs a source to be identified, can be any value
# but if loading from the internet, you'd use the URI
codeobj = compile(modulesource, sourcestr, 'exec')
newmodule = imp.new_module(modname)
exec(codeobj,newmodule.__dict__)
return newmodule
newmodule = makemodule(modulesource)
print(newmodule.a)
At this point newmodule is already a module object in scope, so you don't need to import it or anything.
modulesource = '''
a = 'foo'
def myfun(astr):
return a + astr
'''
newmod = makemodule(modulesource)
print(newmod.myfun('bat'))
Ideone here: http://ideone.com/dXGziO
Tested with python 2, should work with python 3 (textually compatible print used;function-like exec syntax used).

This seems to be a use case for a self-written import hook. Look up in PEP 302 how exactly they work.
Essentially, you'll have to provide a finder object which, in turn, provides a loader object. I don't understand the process at the very first glance (otherwise I'd be more explicit), but the PEP contains all needed details for implementing the stuff.

As glglgl's has it this import hook has been implemented for Python2 and Python3 in a module called httpimport.
It uses a custom finder/loader object to locate resources using HTTP/S.
Additionally, the import_cdn function in Jossef Harush's answer is almost identically implemented in httpimport's github_repo, and bitbucket_repo functions.
#Marcin's answer contains a good portion of the code of the httpimport's loader class.

How to offer platform-specific implementations of a module?

I need to make one function in a module platform-independent by offering several implementations, without changing any files that import it. The following works:
do_it = getattr(__import__(__name__), "do_on_" + sys.platform)
...but breaks if the module is put into a package.
An alternative would be an if/elif with hard-coded calls to the others in do_it().
Anything better?

Put the code for platform support in different files in your package. Then add this to the file people are supposed to import from:
if sys.platform.startswith("win"):
from ._windows_support import *
elif sys.platform.startswith("linux"):
from ._unix_support import *
else:
raise ImportError("my module doesn't support this system")

Use globals()['do_on_' + platform] instead of the getattr call and your original idea should work whether this is inside a package or not.

If you need to create a platform specific instance of an class you should look into the Factory Pattern:
link text

Dive Into Python offers the exceptions alternative.