p4p

4.2.2 verified Sat May 09 auth: no python

Python interface to PVAccess (PV Access) protocol client for EPICS. Current version 4.2.2, supports Python >=2.7. Maintained by EPICS community, follow EPICS release cadence.

pip install p4p

Common errors

error ImportError: cannot import name 'Context' from 'p4p' ↓

cause Using old import path: `from p4p import Context`. Context moved to `p4p.client` in v4.

fix

Change to: from p4p.client import Context

error ModuleNotFoundError: No module named 'p4p.client.rpc' ↓

cause The rpc module was deprecated and removed in p4p 4.2.

fix

Use from p4p.client.pva import Context and use .rpc() method on context instead of separate rpc module.

error TypeError: __init__() got an unexpected keyword argument 'provider' ↓

cause In p4p <4.0, Context accepted `provider` as keyword; later versions use positional first argument.

fix

Use Context('pva') instead of Context(provider='pva').

error p4p.client.pva.ChannelTimeout: Channel timeout ↓

cause PV name not found or network issues; default timeout is 5 seconds.

fix

Check PV name spelling, network connectivity, or increase timeout: ctx.get('pvname', timeout=10)

Warnings

breaking p4p 4.0 removed support for Python 2.7. Users on older systems must pin to p4p<4.0. ↓

fix Use Python 3.6+ or pin to p4p==3.x.

breaking Context usage changed: old import path `from p4p import Context` no longer works; use `from p4p.client import Context`. ↓

fix Update imports to `from p4p.client import Context`.

deprecated The `rpc` module (p4p.client.rpc) is deprecated in favor of new `p4p.client.pva` for all operations. Expect removal in a future major release. ↓

fix Migrate from `from p4p.client import rpc` to `from p4p.client.pva import Context`.

gotcha Many examples use `Context('pva', ...)` but the provider string is environment-specific. Default provider 'pva' works for most sites, but if server uses 'ca' or custom, must specify correctly. ↓

fix Check site's PV Access provider setting (e.g., EPICS_PVA_PROVIDER). Use `Context('pva')` for native PVA, or `Context('ca')` for Channel Access relay.

gotcha Monitoring channels using `ctx.monitor()` returns an iterator that blocks indefinitely. For GUI or non-blocking applications, use `ch = ctx.monitor(...); ch.start()` with callbacks (see docs for `start` method). ↓

fix Use callback-based monitoring: `ch = ctx.monitor(...); ch.start(lambda x: print(x))`

Imports

pva
wrong
```
import p4p
```
correct
```
from p4p.client import pva
```
Direct import of p4p does not expose the client; must import submodule.

wrong

from p4p import PV

correct

from p4p.client.pva import PV

PV class is inside pva submodule.

Channel

wrong

from p4p import Channel

correct

from p4p.client.pva import Channel

Channel class is inside pva submodule.

Context

wrong

from p4p import Context

correct

from p4p.client import Context

Context is inside client submodule.

Quickstart

Create a PVA client context, fetch a value or monitor a PV.

import os
from p4p.client.pva import Context

with Context('pva') as ctx:
    value = ctx.get(os.environ.get('PV_NAME', 'blah:example'))
    print(value)

# Alternatively, using channel for monitoring:
with Context('pva') as ctx:
    ch = ctx.monitor(os.environ.get('PV_NAME', 'blah:example'))
    for update in ch:
        print(update)
        break  # just one sample