python-snap7

3.0.0 · active · verified Thu Apr 16

python-snap7 is a pure Python S7 communication library for Siemens PLCs, implementing the full protocol stack (TPKT, COTP, S7) in Python. As of version 3.0.0, it no longer requires the underlying Snap7 C library. It provides an interface for connecting to and interacting with Siemens S7-300, S7-400, S7-1200, and S7-1500 PLCs. The library is actively maintained with a moderate release cadence, with major versions introducing significant architectural changes.

Common errors

```
ModuleNotFoundError: No module named 'snap7.snap7'
```
cause Attempting to import the old C-binding wrapper module (`snap7.snap7`) which was removed in version 3.0.0.

fix Update your import statements to use the new pure Python client: `from snap7.client import Client`.
```
snap7.snap7exceptions.Snap7Exception: error while connecting: 10060
```
cause This usually indicates a connection timeout or refusal. Common reasons include incorrect IP/rack/slot, PLC not reachable, PLC not configured for PUT/GET communication, or firewall blocking the connection.

fix 1. Verify `plc_ip`, `plc_rack`, `plc_slot` are correct. 2. Ping the PLC's IP address. 3. Ensure PUT/GET communication is enabled in the PLC's TIA Portal project. 4. Check network firewalls on both the client and PLC side.
```
TypeError: 'str' object cannot be interpreted as a buffer
```
cause Passing a string directly to functions expecting a bytearray or bytes object (e.g., `db_write`, `mb_write`).

fix Convert your string data to a `bytearray` or `bytes` object before passing it to the write functions. Example: `data_to_write = bytearray(b'Hello')` or `data_to_write = 'Hello'.encode('ascii')`.

Warnings

breaking Version 3.0.0 is a complete rewrite in pure Python, removing the dependency on the Snap7 C library. This changes internal architecture and some import paths.
Fix: Update your code to use `from snap7.client import Client` instead of `import snap7.snap7` or `from snap7 import snap7`. Remove any manual C library installations (`libsnap7.so`, `snap7.dll`). Consult the v3.0.0 documentation for updated API specifics.
breaking Version 2.0.0 introduced significant code restructuring with potential, though not fully documented, API changes for method arguments and internal types.
Fix: Review the official documentation and changelog for `python-snap7` v2.x to v3.x migration. Pay close attention to method signatures and data type handling, especially if upgrading from pre-2.0 versions.
gotcha Older versions of `python-snap7` (pre-3.0.0) required the Snap7 C shared library (`libsnap7.so` on Linux, `snap7.dll` on Windows) to be installed and accessible in the system's PATH or library search paths.
Fix: For versions prior to 3.0.0, ensure the Snap7 C library is correctly installed for your operating system and its location is known to your environment. For v3.0.0+, this is no longer necessary.
gotcha Siemens PLCs often require specific settings (e.g., 'Permit access with PUT/GET communication from remote partner (PLC, HMI, SCADA)') to allow external communication. Without this, `python-snap7` connections will fail.
Fix: Check your PLC's TIA Portal project settings for the CPU and ensure PUT/GET communication is enabled. Also verify firewall rules on both the PLC network and the client machine.

Install

pip install python-snap7 Install latest version

Imports

Client
wrong:
```
import snap7.snap7
```
correct:
```
from snap7.client import Client
```
As of v3.0.0, the C library wrapper `snap7.snap7` is removed. Use `snap7.client.Client`.

Quickstart

This quickstart demonstrates how to connect to a Siemens PLC using `snap7.client.Client`, read 10 bytes from Data Block (DB) 10, and then disconnect. Ensure the PLC's IP address, rack, and slot are correctly configured. Remember to enable PUT/GET communication on your PLC for the library to function correctly.

import snap7
from snap7.snap7types import S7AreaDB
import os

# Configure PLC connection details
plc_ip = os.environ.get('PLC_IP', '192.168.0.100') # Placeholder IP
plc_rack = int(os.environ.get('PLC_RACK', '0'))
plc_slot = int(os.environ.get('PLC_SLOT', '1'))

client = snap7.client.Client()

try:
    client.connect(plc_ip, plc_rack, plc_slot)
    if client.get_connected():
        print(f"Successfully connected to PLC at {plc_ip}")

        # Example: Read 10 bytes from Data Block (DB) number 10, starting at byte 0
        db_number = 10
        start_byte = 0
        size = 10
        data_buffer = client.db_read(db_number, start_byte, size)
        print(f"Read from DB{db_number}, Start:{start_byte}, Size:{size}: {data_buffer}")

        # Example: Write 10 bytes to Data Block (DB) number 10, starting at byte 0
        # Make sure the PLC is configured to allow writes.
        # new_data = bytearray([0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A])
        # client.db_write(db_number, start_byte, new_data)
        # print(f"Wrote to DB{db_number}, Start:{start_byte}, Size:{size}: {new_data}")

    else:
        print("Failed to connect to PLC.")

except Exception as e:
    print(f"An error occurred: {e}")
finally:
    if client.get_connected():
        client.disconnect()
        print("Disconnected from PLC.")

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