# Patterns and precedence

**URL:** <https://forum.ansible.com/t/patterns-and-precedence/17494>\
**Category:** Ansible Project\
**Created:** [February 10, 2015, 8:47pm UTC](https://forum.ansible.com/t/patterns-and-precedence/17494 "2015-02-10T20:47:56Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Josh\_Smift1](https://avatars.discourse-cdn.com/v4/letter/j/bcef8e/32.png) [@Josh\_Smift1](https://forum.ansible.com/u/Josh_Smift1)\
**Post date:** [February 10, 2015, 8:47pm UTC](https://forum.ansible.com/t/patterns-and-precedence/17494/1 "2015-02-10T20:47:56Z")

</div>

[http://docs.ansible.com/intro\_patterns.html](http://docs.ansible.com/intro_patterns.html) shows

&nbsp;&nbsp;webservers:dbservers:&staging:!phoenix

as an example of a combination of unions and intersections, and says that  
this means

&nbsp;&nbsp;all machines in the groups `webservers' and `dbservers' are to be  
&nbsp;&nbsp;managed if they are in the group `staging' also, but the machines are  
&nbsp;&nbsp;not to be managed if they are in the group `phoenix'

To simplify slightly, because I don't actualy need the negation in my  
example, it's saying that

&nbsp;&nbsp;webservers:dbservers:&staging

means

&nbsp;&nbsp;(websevers UNION dbservers) INTERSECTION staging

which is cool.

But, I have a use case where I want to say

&nbsp;&nbsp;webservers UNION (dbservers INTERSECTION staging)

i.e. I want to manage with all webservers and only staging dbservers. I  
didn't see an obvious way to do that; all three of

&nbsp;&nbsp;webservers:dbservers:&staging  
&nbsp;&nbsp;webservers:staging:&dbservers  
&nbsp;&nbsp;dbservers:&staging:webservers  
&nbsp;&nbsp;staging:&dbservers:webservers

seem to be equivalent, and adding parentheses didn't seem to be supported  
(I got "ERROR: provided hosts list is empty").

I particularly want to use it in a --limit argument, if that matters.

Is there a way to do this, or does union always take precedence over  
intersection?

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-Josh (jbs@care.com)

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---

<div class="post-metadata">

**Author:** ![Giovanni\_Tirloni](https://avatars.discourse-cdn.com/v4/letter/g/71c47a/32.png) [@Giovanni\_Tirloni](https://forum.ansible.com/u/Giovanni_Tirloni)\
**Post date:** [February 13, 2015, 12:44pm UTC](https://forum.ansible.com/t/patterns-and-precedence/17494/2 "2015-02-13T12:44:59Z")

</div>

There magic happens in lib/ansible/inventory.get\_hosts() [1].

# when applying the host selectors, run those without the "&" or "!"  
# first, then the &s, then the !s.  
patterns = pattern\_regular + pattern\_intersection + pattern\_exclude

[...]

if p.startswith("!"):  
&nbsp;&nbsp;hosts = [h for h in hosts if h not in that]  
elif p.startswith("&"):  
&nbsp;&nbsp;hosts = [h for h in hosts if h in that]  
else:  
&nbsp;&nbsp;to\_append = [ h for h in that if h.name not in [ y.name for y in hosts  
&nbsp;&nbsp;] ]  
&nbsp;&nbsp;hosts.extend(to\_append)

My brain can't find a way to define what you want using patterns alone.

Filled issue#10234 as a possible improvement [2].

[1] -  
[https://github.com/ansible/ansible/blob/devel/lib/ansible/inventory/\_\_init\_\_.py#L176](https://github.com/ansible/ansible/blob/devel/lib/ansible/inventory/ __init__.py#L176)  
[2] - [https://github.com/ansible/ansible/issues/10234](https://github.com/ansible/ansible/issues/10234)

Giovanni