I can't reach my sht21 sensor throughout the Multiplexer tca9548a (adafruit). When I run it with python3 in the raspberry pi's terminal, I get this error report:

Traceback (most recent call last):
  File "sensormeasurement.py", line 15, in <module>
    sensor1 = sht21.SHT21(tca[0])
  File "/home/pi/programming_stuff/sht21.py", line 20, in __init__
    self.bus.write_byte(self._I2C_ADDRESS, self._SOFTRESET)
AttributeError: 'TCA9548A_Channel' object has no attribute 'write_byte'

I think, this is because I mixed the code from adafruit blinka: Github repository Adafruit_CircuitPython_TCA9548A

# The MIT License (MIT)
#
# Copyright (c) 2018 Carter Nelson for Adafruit Industries
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.
"""
``Adafruit_TCA9548A``
====================================================

CircuitPython driver for the TCA9548A I2C Multiplexer.

* Author(s): Carter Nelson

Implementation Notes
--------------------

**Hardware:**

* TCA9548A I2C Multiplexer: https://www.adafruit.com/product/2717

**Software and Dependencies:**

* Adafruit CircuitPython firmware for the supported boards:
  https://github.com/adafruit/circuitpython/releases
* Adafruit's Bus Device library: https://github.com/adafruit/Adafruit_CircuitPython_BusDevice
"""

# imports
from micropython import const

_DEFAULT_ADDRESS = const(0x70)

__version__ = "0.0.0-auto.0"
__repo__ = "https://github.com/adafruit/Adafruit_CircuitPython_TCA9548A.git"

class TCA9548A_Channel():
    """Helper class to represent an output channel on the TCA9548A and take care
       of the necessary I2C commands for channel switching. This class needs to
       behave like an I2CDevice."""

    def __init__(self, tca, channel):
        self.tca = tca
        self.channel_switch = bytearray([1 << channel])

    def try_lock(self):
        """Pass thru for try_lock."""
        while not self.tca.i2c.try_lock():
            pass
        self.tca.i2c.writeto(self.tca.address, self.channel_switch)
        return True

    def unlock(self):
        """Pass thru for unlock."""
        return self.tca.i2c.unlock()

    def readfrom_into(self, address, buffer, **kwargs):
        """Pass thru for readfrom_into."""
        if address == self.tca.address:
            raise ValueError("Device address must be different than TCA9548A address.")
        return self.tca.i2c.readfrom_into(address, buffer, **kwargs)

    def writeto(self, address, buffer, **kwargs):
        """Pass thru for writeto."""
        if address == self.tca.address:
            raise ValueError("Device address must be different than TCA9548A address.")
        return self.tca.i2c.writeto(address, buffer, **kwargs)


class TCA9548A():
    """Class which provides interface to TCA9548A I2C multiplexer."""

    def __init__(self, i2c, address=_DEFAULT_ADDRESS):
        self.i2c = i2c
        self.address = address
        self.channels = [None]*8

    def __len__(self):
        return 8

    def __getitem__(self, key):
        if not 0 <= key <= 7:
            raise IndexError("Channel must be an integer in the range: 0-7")
        if self.channels[key] is None:
            self.channels[key] = TCA9548A_Channel(self, key)
        return self.channels[key]

with a program for the i2c triggered sht21 sensor from this repository modified for python3:

import smbus 
import time

class SHT21:
    """Class to read temperature and humidity from SHT21.
    Ressources: 
      http://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/Humidity/Sensirion_Humidity_SHT21_Datasheet_V3.pdf
      https://github.com/jaques/sht21_python/blob/master/sht21.py
      Martin Steppuhn's code from http://www.emsystech.de/raspi-sht21"""

    #control constants
    _SOFTRESET = 0xFE
    _I2C_ADDRESS = 0x40
    _TRIGGER_TEMPERATURE_NO_HOLD = 0xF3
    _TRIGGER_HUMIDITY_NO_HOLD = 0xF5

    def __init__(self, bus):
        """According to the datasheet the soft reset takes less than 15 ms."""
        self.bus = bus
        self.bus.write_byte(self._I2C_ADDRESS, self._SOFTRESET)
        time.sleep(0.015)

    def read_temperature(self):    
        """Reads the temperature from the sensor.  Not that this call blocks
    for 250ms to allow the sensor to return the data"""
        data = []
        self.bus.write_byte(self._I2C_ADDRESS, self._TRIGGER_TEMPERATURE_NO_HOLD)
        time.sleep(0.250)
        data.append(self.bus.read_byte(self._I2C_ADDRESS))
        data.append(self.bus.read_byte(self._I2C_ADDRESS))
        return self._get_temperature_from_buffer(data)


    def read_humidity(self):    
        """Reads the humidity from the sensor.  Not that this call blocks 
    for 250ms to allow the sensor to return the data"""
        data = []
        self.bus.write_byte(self._I2C_ADDRESS, self._TRIGGER_HUMIDITY_NO_HOLD)
        time.sleep(0.250)
        data.append(self.bus.read_byte(self._I2C_ADDRESS))
        data.append(self.bus.read_byte(self._I2C_ADDRESS))
        return self._get_humidity_from_buffer(data)    

    def _get_temperature_from_buffer(self, data):
        """This function reads the first two bytes of data and 
        returns the temperature in C by using the following function:
        T = =46.82 + (172.72 * (ST/2^16))
        where ST is the value from the sensor
        """
        unadjusted = (data[0] << 8) + data[1]
        unadjusted *= 175.72
        unadjusted /= 1 << 16 # divide by 2^16
        unadjusted -= 46.85
        return unadjusted

    def _get_humidity_from_buffer(self, data):
        """This function reads the first two bytes of data and returns 
        the relative humidity in percent by using the following function:
        RH = -6 + (125 * (SRH / 2 ^16))
        where SRH is the value read from the sensor
        """
        unadjusted = (data[0] << 8) + data[1]
        unadjusted *= 125
        unadjusted /= 1 << 16 # divide by 2^16
        unadjusted -= 6
        return unadjusted



    def close(self):
        """Closes the i2c connection"""
        self.bus.close()


    def __enter__(self):
        """used to enable python's with statement support"""
        return self


    def __exit__(self, type, value, traceback):
        """with support"""
        self.close()


if __name__ == "__main__":
    try:
        bus = smbus.SMBus(1)
        with SHT21(bus) as sht21:
            print "Temperature: %s"%sht21.read_temperature()
            print "Humidity: %s"%sht21.read_humidity()
    except IOError, e:
        print e
print 'Error creating connection to i2c.'

in the modified document (which is also from Github repository Adafruit_CircuitPython_TCA9548A) which I named sensormeasurement.py:

# This program is using Senirion SHT21 sensors for humidity and temperature measurement which is attached to TCA9548A Multiplexer through channels 0 and 1. This is like the example in https://github.com/adafruit/Adafruit_CircuitPython_TCA9548A 
import time
import board
import busio
import adafruit_tca9548a
import sht21

# Create I2C bus as normal
i2c = busio.I2C(board.SCL, board.SDA)

# Create the TCA9548A object and give it the I2C bus
tca = adafruit_tca9548a.TCA9548A(i2c)

# For each sensor, create it using the TCA9548A channel instead of the I2C object
sensor1 = sht21.SHT21(tca[0])
sensor2 = sht21.SHT21(tca[1])
sensor3 = sht21.SHT21(tca[2])

# Loop and profit!
while True:
    print ("Temperature: %s" % sensor1.read_temperature())
    print ("Humidity: %s" % sensor1.read_humidity())
    time.sleep(0.1)

I installed CiruitPython, and when I rewire my circuit, so that the sensor is directly plugged in the i2c port of the raspberry pi, then the sht21.py runs perfectly. But since I what to use more than one sensor, I want to use the multiplexer. Maybe the problem is that I use smbus in sht21.py and in sensormeasurement.py I used busio instead. So, how should I modify sht21.py? I am new at this subject, so please explain it in detail. Thanks.

up vote 1 down vote accepted

I figured it out with a little help: Circuit Python doesn't work with this sht21.py, because it is smbus based. So here it is: I used the multiplexer code from this homepage but this is all I needed

# TCA9548A I2C multiplexer
# I2C Address: 70 through 77
# Channel: 0 - 7

import smbus

# class for the I2C switch 
class TCA9548A(object):
    # init
    def __init__(self,name,address,bus_nr):
        self.name=name
        self.address=address
        self.bus_nr=bus_nr
        self.bus=smbus.SMBus(bus_nr) # for bus_nr = /def/i2c-bus_nr

    # Change to i2c channel 0..7
    def chn(self,channel):
        self.bus.write_byte(self.address,2**channel)

And then I adjusted the sensormeasurement.py:

import time
import multiplexer
import sht21
import smbus 

# I2C bus already in multiplexer.py created

# Create the TCA9548A object and give it the I2C bus
tca = multiplexer.TCA9548A('I2C switch 0',0x70,1)

# For each sensor, create it using the TCA9548A channel instead of the I2C object
for x in [0, 1, 2]:
    tca.chn(x)
    sensor = sht21.SHT21(tca.bus)
    print ("Temperature: %s" % sensor.read_temperature())
    print ("Humidity: %s" % sensor.read_humidity())

So, perhaps this will help someone in the future.

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