The Ansible module development documentation states:
Key parts [of writing an Ansible module] include always ending the module file with:
from ansible.module_utils.basic import *
main()
This contradicts the usual practice of grouping imports at the top of the file. Using import * also prevents lint tools (e.g. flake8) from working effectively, and is generally regarded as bad practice.
Is there any reason to import in this way, or is Ansible just making their own style recommendation here?
NOTE: The below answer no longer pertains to Ansible 2.1+. From the comments:
I realize this is an old post but should anyone still be interested, it's worth noting that this is not true anymore since ansible 2.1. Taken from here: Prior to Ansible-2.1.0, importing only what you used from ansible.module_utils.basic did not work. You needed to use a wildcard import - bouletta
Original Answer
Ansible (prior to version 2.1) will refuse to run if you don't do the import * business. I'm not 100% certain what magic is being done, but I know some is.
The Replacer is used to insert chunks of code into modules before
transfer. Rather than doing classical python imports, this allows for more
efficient transfer in a no-bootstrapping scenario by not moving extra files
over the wire, and also takes care of embedding arguments in the transferred
modules.
This version is done in such a way that local imports can still be
used in the module code, so IDEs don't have to be aware of what is going on.
Example:
from ansible.module_utils.basic import *
... will result in the insertion basic.py into the module
from the module_utils/ directory in the source tree.
All modules are required to import at least basic, though there will also
be other snippets.
Related
Because the main file was getting quite long, I decided to split my code into two files, one containing all functions (here called common_functions.py) and one containing classes and methods, the later importing the former. The issue is, I would like to import those functions using
from common_functions import *
as I do not need a prefix for the function inside and because they are quite numerous. Also, although not important, this allows me not to have to repeat importing packages in the class file. The issue is, spyder does not identify the inside of the imported file, resulting in warnings everywhere as shown below, although the code is executed normally.
So my question is, is there a way to remove these warning, either by explaining spyder how to get to the packages and functions of common_functions.py, or to organising the code in a different way ?
Spyder uses pyflakes under the hood for the real-time code analysis in the editor pane. Pyflakes doesn't have the capability to interpret wildcard import statements (i.e. it won't retrieve all the names that are actually imported by a wildcard import). Hence the warning messages about undefined names that you're getting.
I would recommend to avoid wildcard imports altogether in your code files. Though wildcard imports are valid python code, they are widely considered a bad practice in most situations (see here and here for more detailed explanation). If you replace from common_functions import * with for example import common_functions as cf, then the prefix that you have to use is minimal in length.
Python is extremely elegant language. Well, except... except imports. I still can't get it work the way it seems natural to me.
I have a class MyObjectA which is in file mypackage/myobjecta.py. This object uses some utility functions which are in mypackage/utils.py. So in my first lines in myobjecta.py I write:
from mypackage.utils import util_func1, util_func2
But some of the utility functions create and return new instances of MyObjectA. So I need to write in utils.py:
from mypackage.myobjecta import MyObjectA
Well, no I can't. This is a circular import and Python will refuse to do that.
There are many question here regarding this issue, but none seems to give satisfactory answer. From what I can read in all the answers:
Reorganize your modules, you are doing it wrong! But I do not know
how better to organize my modules even in such a simple case as I
presented.
Try just import ... rather than from ... import ...
(personally I hate to write and potentially refactor all the full
name qualifiers; I love to see what exactly I am importing into
module from the outside world). Would that help? I am not sure,
still there are circular imports.
Do hacks like import something in the inner scope of a function body just one line before you use something from other module.
I am still hoping there is solution number 4) which would be Pythonic in the sense of being functional and elegant and simple and working. Or is there not?
Note: I am primarily a C++ programmer, the example above is so much easily solved by including corresponding headers that I can't believe it is not possible in Python.
There is nothing hackish about importing something in a function body, it's an absolutely valid pattern:
def some_function():
import logging
do_some_logging()
Usually ImportErrors are only raised because of the way import() evaluates top level statements of the entire file when called.
In case you do not have a logic circular dependency...
, nothing is impossible in python...
There is a way around it if you positively want your imports on top:
From David Beazleys excellent talk Modules and Packages: Live and Let Die! - PyCon 2015, 1:54:00, here is a way to deal with circular imports in python:
try:
from images.serializers import SimplifiedImageSerializer
except ImportError:
import sys
SimplifiedImageSerializer = sys.modules[__package__ + '.SimplifiedImageSerializer']
This tries to import SimplifiedImageSerializer and if ImportError is raised (due to a circular import error or the it not existing) it will pull it from the importcache.
PS: You have to read this entire post in David Beazley's voice.
Don't import mypackage.utils to your main module, it already exists in mypackage.myobjecta. Once you import mypackage.myobjecta the code from that module is being executed and you don't need to import anything to your current module, because mypackage.myobjecta is already complete.
What you want isn't possible. There's no way for Python to know in which order it needs to execute the top-level code in order to do what you ask.
Assume you import utils first. Python will begin by evaluating the first statement, from mypackage.myobjecta import MyObjectA, which requires executing the top level of the myobjecta module. Python must then execute from mypackage.utils import util_func1, util_func2, but it can't do that until it resolves the myobjecta import.
Instead of recursing infinitely, Python resolves this situation by allowing the innermost import to complete without finishing. Thus, the utils import completes without executing the rest of the file, and your import statement fails because util_func1 doesn't exist yet.
The reason import myobjecta works is that it allows the symbols to be resolved later, after the body of every module has executed. Personally, I've run into a lot of confusion even with this kind of circular import, and so I don't recommend using them at all.
If you really want to use a circular import anyway, and you want them to be "from" imports, I think the only way it can reliably work is this: Define all symbols used by another module before importing from that module. In this case, your definitions for util_func1 and util_func2 must be before your from mypackage.myobjecta import MyObjectA statement in utils, and the definition of MyObjectA must be before from mypackage.utils import util_func1, util_func2 in myobjecta.
Compiled languages like C# can handle situations like this because the top level is a collection of definitions, not instructions. They don't have to create every class and every function in the order given. They can work things out in whatever order is required to avoid any cycles. (C++ does it by duplicating information in prototypes, which I personally feel is a rather hacky solution, but that's also not how Python works.)
The advantage of a system like Python is that it's highly dynamic. Yes you can define a class or a function differently based on something you only know at runtime. Or modify a class after it's been created. Or try to import dependencies and go without them if they're not available. If you don't feel these things are worth the inconvenience of adhering to a strict dependency tree, that's totally reasonable, and maybe you'd be better served by a compiled language.
Pythonistas frown upon importing from a function. Pythonistas usually frown upon global variables. Yet, I saw both and don't think the projects that used them were any worse than others done by some strict Pythhonistas. The feature does exist, not going into a long argument over its utility.
There's an alternative to the problem of importing from a function: when you import from the top of a file (or the bottom, really), this import will take some time (some small time, but some time), but Python will cache the entire file and if another file needs the same import, Python can retrieve the module quickly without importing. Whereas, if you import from a function, things get complicated: Python will have to process the import line each time you call the function, which might, in a tiny way, slow your program down.
A solution to this is to cache the module independently. Okay, this uses imports inside function bodies AND global variables. Wow!
_MODULEA = None
def util1():
if _MODULEA is None:
from mymodule import modulea as _MODULEA
obj = _MODULEA.ClassYouWant
return obj
I saw this strategy adopted with a project using a flat API. Whether you like it or not (and I'm not sure about that myself), it works and is fast, because the import line is executed only once (when the function first executes). Still, I would recommend restructuring: problems with circular imports show a problem in structure, usually, and this is always worth fixing. I do agree, though, it would be nice if Python provided more useful errors when this kind of situation happens.
If my code uses third party modules that cannot be trusted, is there anything to prevent situation like this:
UntrustedModule.py:
import random
random.random = lambda : 4
MyModule.py:
import random
import UntrustedModule
print (random.random())
where just importing this module breaks assumptions about other, unrelated ones?
No, you can't have any such guarantee in Python, at least not in the CPython implementation. When you import a module its code is run, and it has full access to every part of the interpreter (and likely big parts of your system). No way to avoid this. It is unwise to ever load untrusted code, because there is so much it can do.
However you may be interested in running the process in an isolated process, and only communicate with it by IPC. This is a huge topic and it depends on the degree of isolation you need and how much you trust the external code.
PyPy implements some sandboxing features. It's not as simple as just "turning sandboxing on" but it's one of many ways to isolate untrusted code.
You can do a
reload(random)
in order to reload it from source resp. to restore it as it is intended to be.
Python will import in the order of searching the local path (the directory from which the script was executed) first, then any paths listed in the PYTHONPATH environment variable.
A better solution would be to inspect and write tests against your untrusted module.
I am relatively quite new to Python and I try to learn the "Pythonic" way of doing things to build a solid foundation in terms of Python development. Perhaps what I want to achieve is not Python at all, but I am nonetheless seeking to find out the "right" way to solve this issue.
I am building an application, for which I am creating modules. I just noticed that a module of mine has 7 different .py Python files, all importing 3 different same things. So all these files share these imports.
I tried removing them, and inserting these import to the empty init.py in the folder, but it did not do the trick.
If possible, since these imports are needed by all these module files, I would not like them to be imported in each file one by one.
What can I do to perform the common import?
Thank you very much, I really appreciate your kind help.
As the Zen of Python states, "Explicit is better than implicit", and this is a good example.
It's very useful to have the dependencies of a module listed explicitly in the imports and it means that every symbol in a file can be traced to its origin with a simple text search. E.g. if you search for some_identifier in your file, you'll either find a definition in the file, or from some_module import some_identifier. It's even more obvious with direct references to some_module.some_identifier. (This is also one reason why you should not do from module import *.)
One thing you could do, without losing the above property, is to import your three shared modules into a fourth module:
#fourth.py
import first
import second
import third
then...
#another.py
import fourth
fourth.first.some_function()
#etc.
If you can't stomach that (it does make calls more verbose, after all) then the duplication of three imports is fine, really.
I agree with DrewV, it is perfectly pythonic to do
File1:
import xyz
import abc
...
File2:
import xyz
An almost identical question has also been addressed in the following link:
python multiple imports for a common module
As it explains, Python does the job of optimising the module load, so you can write multiple import statements and not worry about performance losses, because the module is only loaded once. In fact, listing out all the imports in each file makes it explicitly clear what each file depends on.
And for a discussion of how imports interact with namespaces, see:
Python imports across modules and global variables
I have this core python module we use in our facility called mfxLib. I need to be able to keep different version of this module without breaking all the other modules/plugin that are importing this module.
My solution was keep a duplicate of my module by renaming them mfxLib01 and mfxLib02 then
to replace the original mfxLib module with an empty module containing only a __init__.py file that import the latest version.
# content of mfxLib.__init__.py
from mfxLib02 import *
This seems logical and seems to work but I was wondering if there was a common practice for doing this? guidelines to follow? etc
Thanks
You can import a module as another name. Commonly people use this to save typing in a long module name, for example:
import numpy as np
np.array([1,2,3,4])
Hence you could do:
import mfxLib01 as mfxLib
or
import mfxLib02 as mfxLib
then your code uses mfxLib everywhere.
That might help...
If you have different scripts requiring different versions, your current approach should be the the best, but I'd suggest using a version control system like Git or SVN. That would allow you to commit and revert to earlier versions easily, as well as share the module with other users.
Version control will almost certainly make your life easier. In addition to Petterson's recommendations, consider Mercurial. Like git and SVN, it's free. It's also written in Python and should run without difficulty on any of your systems.
Spacedman's recommendations are also useful, especially if the differences between the versions represent customizations for particular systems and the customizations are relatively stable. Note that you can use that approach in combination with a version control system.
Finally, it's always worthwhile to make a strong effort to write your module so that it can work without modification everywhere. Often, you can accomplish this by adding some optional arguments to a few key functions to handle the different requirements. Python is really convenient in that regard because keyword arguments at the end of the arg list are always optional, so you can easily arrange to provide the existing behavior by giving them suitable default values.
def foo(oldarg1, oldarg2, newarg1=None):
if newarg1 != None:
## behave differently
else:
## behave as usual