Re: [Fab-user] New to fabric 2. Using groups with Connection?

[Christian MacNevin]

Thanks so much for the epic response 😊 So far Fabric 2 seems more sensible in a lot of ways, and since my old codebase was so integrated into a former employer’s infrastructure, I’m going to keep trying to operate in the new world.

I’ll work through the workaround for now. When is 2.3 likely to drop?

 

 

 

From: address@hidden <address@hidden> On Behalf Of Jeff Forcier
Sent: Monday, July 23, 2018 9:56 PM
To: Christian MacNevin <address@hidden>
Cc: address@hidden
Subject: Re: [Fab-user] New to fabric 2. Using groups with Connection?

 

Hi Christian,

 

First, do note you can always fall back to Fabric 1 for a while if Fabric 2 isn't yet up to snuff for your particular use case. We're still feeling out some of the APIs in terms of ease of use. Unfortunately, Group is one of those - it's real basic right now.

 

I just looked and there's no great way to set global connect_kwargs in such an object, without creating your own Connections to hand to its alternate constructor 'from_connections()':

 

```

hosts = ['switch1', 'switch2', 'switch3']

connections = [Connection(host=host, user='user', connect_kwargs={'password': 'pw'}) for host in hosts]

switch_group = group.SerialGroup.from_connections(connections)

 

@task

def inventory(c):

  switch_group.run('show chassis hardware')

```

 

That's not the end of the world, but it's not ideal, so I just made https://github.com/fabric/fabric/issues/1831 and then (since I was thinking about it and it was very easy for once) implemented it. It'll be out in the next feature release!

 

The rest of your issues are more high level, so here's a bit of a wall of text (sorry!):

 

Version 2 is much less implicit/magic than v1 – all objects need to fit together in obvious, Pythonic ways (such as being handed to one another in constructors or method calls). For example, context managers don't change any object besides the one that they yield, so trying to do things to switch_group inside a context manager about some other Connection object won't do anything. (This is as opposed to e.g. 'with settings()' in v1, which I assume you were thinking of; it, like the rest of that API, is all about magically twiddling data behind the scenes.)

 

Connections can't be created without some 'host' argument, which I think is triggering your TypeError. This ought to be in its __init__ docs, FWIW :) See the code example above; you either need to make your Group from explicitly-created Connections, where a host arg is being given, or (after the next release) you can tell the Group's constructor about connect_kwargs.

 

Depending on your use case, there's a CLI flag that may replace your getpass code: http://docs.fabfile.org/en/2.2/cli.html#cmdoption-prompt-for-login-password

 

Which brings us to that context argument in the task signature: that's how `fab` explicitly transmits CLI and config info to your tasks, including the connection password. However, this introduces some ordering problems with your code - you need to pass that context's config into your Group or Connections.

 

Should be easily solved by moving away from module-level code exec (which is honestly an antipattern anyways). Based on my above snippet:

 

```

hosts = ['switch1', 'switch2', 'switch3']

 

def make_group(c):

  connections = [

    Connection(host=host, user='user', config=c.config)

    for host in hosts

  ]

  return group.SerialGroup.from_connections(connections)

 

@task

def inventory(c):

  switch_group = make_group(c)

  switch_group.run('show chassis hardware')

```

 

Or, after 2.3 comes out, you could arguably nix make_group() again, though hopefully you can see the various opportunities for refactoring, given that everything is explicit:

 

```

hosts = ['switch1', 'switch2', 'switch3']

 

@task

def inventory(c):

  switch_group = SerialGroup(*hosts, config=c.config)

  switch_group.run('show chassis hardware')

```

 

Finally: if you're thinking "ok, but it'd be so nice to just plop my switch hostnames and the fact that I want password prompting all the time, into a config file and call it done": that sort of thing is definitely in the pipeline! As I said up top, we're still hashing out a lot of the 'convenience' angles here, even though the core APIs are mostly in good shape.

 

Hope that all helps,

Jeff

 

 

On Mon, Jul 23, 2018 at 3:45 PM, Christian MacNevin <address@hidden> wrote:

Hi all,

I’m new to Fabric 2, though in the past I’d used 1 a lot. I can find docs on using SerialGroup, but only with run directly. I’m using interactive mode authentication, so it seems like I need to gather a password (that’s fine, I’m doing it via getpass) and also pass kwargs into Connection.  I have a large list of hosts, and also was planning to use @task decorators.

 

What I have right now is a muddle between the two which obviously won’t work, but I can’t figure out how to glue the three
concepts of having a defined group, a task which can be specified from the cli, and an interactive user/pass sequence together.

 

 

import getpass

from fabric import Connection, group

from invoke import task

 

switch_group = group.SerialGroup(switch1', 'switch2', 'switch3')

 

pw = getpass.getpass("Password for netuser?")

 

@task

def inventory(c):    ß This is purely here because tasks will error out if a kwarg ‘Context(..?)’ isn’t specified

    with Connection(user = 'user', connect_kwargs = {'password' : pw }) as c:

        switch_group.run('show chassis hardware')

 

It’s failing out with ’TypeError: __init__() takes at least 2 arguments (3 given)’

 

 

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--

Jeff Forcier
Unix sysadmin; Python engineer
http://bitprophet.org