@Jens-Halsberghe the actual problem was just the assignment of ohlc values in init:

        self.ohlc.open = self.datas[0].open[0]
        self.ohlc.high = self.datas[0].high[0]
        self.ohlc.low = self.datas[0].low[0]
        self.ohlc.close = self.datas[0].close[0]

@ab_trader there are a lot of shortcuts to different lines, which are not used that much in examples, so it would only make sense to use them, when you know what you are doing.

Thats why your statement is mostly the way to go to use self.lines.x (which also is more readable in code)

@ab_trader here: https://www.backtrader.com/docu/inddev/

Let’s recall that self.lines.dummyline is the long notation and that it can be shortened to:

self.l.dummyline

and even to:

self.dummyline

The latter being only possible if the code has not obscured this with a member attribute.

here is a cleaned up version of the indicator.

Trend_Indicator(bt.Indicator):
    lines = ('trend',)
    params = (
        ('slow_ema', 21),
        ('fast_ema', 8),
    )

    def __init__(self):

        # Add Bollinger bands
        self.bb = bt.indicators.BollingerBands()

        self.slow_ema = bt.indicators.EMA(period=self.params.slow_ema)
        self.fast_ema = bt.indicators.EMA(period=self.params.fast_ema)

        self.ohlc = {"o": None, "h": None, "l": None, "c": None}

        self.trend_bollinger_touch = 0
        self.trend_status = 0
        self.trend_enter = 0
        self.trend_exit_params = {
            "20BarRule": False,
            "EMARule": False,
            "OppositeBollingerTouch": False}
        self.trend_ema_rule = {
            "Close1": False,
            "Close2": False}
        self.trend_position = 0
        self.trend_test = 0

    def nextstart(self):
        self._update_ohlc()

    def next(self):
        if (self.trend_bollinger_touch == 1
                and self.data.low[0] > self.bb.lines.bot[0]
                and self.trend_enter == 0):
            # if the upper bollinger band was touched and the candle
            # we have now makes the highest high and highest close,
            # we enter in an uptrend
            if (self.data.high[0] > self.ohlc["h"]
                    and self.data.close[0] > self.ohlc["c"]):
                self._update_ohlc(["h"])
                self.trend_enter = 1
                self.trend_status = 1
                self.trend_exit_params["OppositeBollingerTouch"] = False
                self.trend_exit_params["20BarRule"] = False
                self.trend_exit_params["EMARule"] = False
                self.trend_position = len(self.ohlc)
                self.trend_test = len(self.data.high)
            if self.data.high[0] > self.ohlc["h"]:
                self._update_ohlc(["h"])
            if self.data.close[0] > self.ohlc["c"]:
                self._update_ohlc(["c"])

        # we enter in a downtrend
        elif (self.trend_bollinger_touch == -1
                and self.data.high[0] < self.bb.lines.top[0]
                and self.trend_enter == 0):
            # if the bottom bollinger band was touched and the candle we
            # have now makes the lowest low and lowest close, we enter
            # in a downtrend
            if (self.data.low[0] < self.ohlc["l"]
                    and self.data.close[0] < self.ohlc["c"]):
                self._update_ohlc(["l"])
                self.trend_enter = -1
                self.trend_status = -1
                self.trend_exit_params["OppositeBollingerTouch"] = False
                self.trend_exit_params["20BarRule"] = False
                self.trend_exit_params["EMARule"] = False
                self.trend_position = len(self.ohlc)
                self.trend_test = len(self.data.low)
            if self.data.low[0] < self.ohlc["l"]:
                self._update_ohlc(["l"])
            if self.data.close[0] < self.ohlc["c"]:
                self._update_ohlc(["c"])

        elif self.trend_bollinger_touch == 0:
            # look for a top bollinger touch, then store that bar's data
            if (self.data.high[0] >= self.bb.lines.top[0]
                    and self.data.low[0] > self.bb.lines.bot[0]):
                self._update_ohlc()
                self.trend_bollinger_touch = 1

            elif (self.data.low[0] <= self.bb.lines.bot[0]
                    and self.data.high[0] < self.bb.lines.top[0]):
                self._update_ohlc()
                self.trend_bollinger_touch = -1

        # if the upper bollinger band was touched but now the lower bollinger
        # band got touched, the upper bollinger touch resets
        if (self.trend_bollinger_touch == 1
                and self.data.low[0] <= self.bb.lines.bot[0]):
            self._update_ohlc()
            self.trend_bollinger_touch = -1

        # if the bottom bollinger band was touched but now the top bollinger
        # band got touched, the bottom bollinger touch resets
        elif (self.trend_bollinger_touch == -1
                and self.data.high[0] >= self.bb.lines.top[0]):
            self._update_ohlc()
            self.trend_bollinger_touch = 1

        # we are in an uptrend
        if self.trend_status == 1:
            # if the opposite bollinger band was touched, we exit the uptrend
            if self.data.low[0] <= self.bb.lines.bot[0]:
                self.trend_status = 0
                self.trend_enter = 0
                self.trend_exit_params["OppositeBollingerTouch"] = True
                self.trend_bollinger_touch = -1
                self.trend_ema_rule['Close1'] = False
                self.trend_ema_rule['Close2'] = False
                self._update_ohlc()
            # if a fresh high is made, keep track of the position
            if self.data.high[0] > self.ohlc["h"]:
                self._update_ohlc(["h"])
                self.trend_position = len(self.data.high)
            # after 20 bars (excluding the first) no new high is made, we are
            # not in a trend anymore
            elif self.data.high[0] < self.ohlc["h"]:
                if len(self.data.high) - self.trend_position >= 21:
                    self.trend_status = 0
                    self.trend_enter = 0
                    self.trend_bollinger_touch = -1
                    self.trend_exit_params["20BarRule"] = True
                    self.trend_ema_rule['Close1'] = False
                    self.trend_ema_rule['Close2'] = False
                    self.trend_exit_params['20BarRule'] = True

            # if the top bollinger band was touched, reset the EMA rule
            if self.data.high[0] >= self.bb.lines.top[0]:
                self.trend_ema_rule['Close1'] = False
                self.trend_ema_rule['Close2'] = False

            if (self.data.close[0] < self.slow_ema[0]
                    and self.data.close[0] < self.fast_ema[0]):
                self.trend_ema_rule['Close1'] = True

            if (self.trend_ema_rule['Close1'] is True
                    and self.data.close[0] > self.slow_ema[0]
                    and self.data.close[0] > self.fast_ema[0]):
                self.trend_ema_rule['Close2'] = True

            if (self.trend_ema_rule['Close1'] is True
                    and self.trend_ema_rule['Close2'] is True
                    and self.data.close[0] < self.slow_ema[0]
                    and self.data.close[0] < self.fast_ema[0]):
                self.trend_ema_rule['Close1'] = False
                self.trend_ema_rule['Close2'] = False
                self.trend_exit_params['EMARule'] = True
                self.trend_enter = 0
                self.trend_status = 0
                self.trend_bollinger_touch = 0

        # we are in a downtrend
        if self.trend_status == -1:
            # if the opposite bollinger band was touched, we exit the downtrend
            if self.data.high[0] >= self.bb.lines.top[0]:
                self.trend_status = 0
                self.trend_enter = 0
                self.trend_bollinger_touch = 1
                self.trend_exit_params["OppositeBollingerTouch"] = True
                self.trend_ema_rule['Close1'] = False
                self.trend_ema_rule['Close2'] = False
                self._update_ohlc()

            # if a fresh low is made, keep track of the position
            if self.data.low[0] < self.ohlc["h"]:
                self._update_ohlc(["l"])
                self.trend_position = len(self.data.low)
            # after 20 bars (excluding the first) no new low is made, we are
            # not in a trend anymore
            elif self.data.low[0] > self.ohlc["l"]:
                if len(self.data.low) - self.trend_position >= 21:
                    self.trend_status = 0
                    self.trend_enter = 0
                    self.trend_bollinger_touch = 0
                    self.trend_exit_params["20BarRule"] = True
                    self.trend_ema_rule['Close1'] = False
                    self.trend_ema_rule['Close2'] = False
                    self.trend_exit_params['20BarRule'] = True

            # if the bottom bollinger band was touched, reset the EMA rule
            if self.data.low[0] <= self.bb.lines.bot[0]:
                self.trend_ema_rule['Close1'] = False
                self.trend_ema_rule['Close2'] = False

            if (self.data.close[0] > self.slow_ema[0]
                    and self.data.close[0] > self.fast_ema[0]):
                self.trend_ema_rule['Close1'] = True

            if (self.trend_ema_rule['Close1'] is True
                    and self.data.close[0] < self.slow_ema[0]
                    and self.data.close[0] < self.fast_ema[0]):
                self.trend_ema_rule['Close2'] = True

            if (self.trend_ema_rule['Close1'] is True
                    and self.trend_ema_rule['Close2'] is True
                    and self.data.close[0] > self.slow_ema[0]
                    and self.data.close[0] > self.fast_ema[0]):
                self.trend_status = 0
                self.trend_enter = 0
                self.trend_bollinger_touch = 0
                self.trend_ema_rule['Close1'] = False
                self.trend_ema_rule['Close2'] = False
                self.trend_exit_params['EMARule'] = True

        # update line
        if (self.trend_enter == -1
                and True not in self.trend_exit_params.values()):
            self.lines.trend[0] = -1
        elif (self.trend_enter == 1
                and True not in self.trend_exit_params.values()):
            self.lines.trend[0] = 1

    def _update_ohlc(self, values=["o", "h", "l", "c"]):
        if "o" in values:
            self.ohlc["o"] = self.data.open[0]
        if "h" in values:
            self.ohlc["h"] = self.data.high[0]
        if "l" in values:
            self.ohlc["l"] = self.data.low[0]
        if "c" in values:
            self.ohlc["c"] = self.data.close[0]

@ab_trader it would be good practice, i assume. I prefer also to use self.lines.x . But since backtrader creates these shortcuts, why not use them?

So here is your fixed code, but to say it again, the way you set the values is confusing.

You create a indicator line ohlc, then you set attributes to it, you never fill it with any data. Also you set attributes to the Trend line to keep track of the trend state.

Better would be not adding the ohlc line, set all your tracking vars to the indicator.

import backtrader as bt


class Trend_Indicator(bt.Indicator):
    lines = ('Trend', 'ohlc')
    params = (
        ('slowma', 21),
        ('fastma', 8),
    )

    def __init__(self):

        # Add Bollinger bands
        self.BB = bt.indicators.BollingerBands(self.datas[0])

        self.SlowEMA = bt.indicators.EMA(
            self.datas[0], period=self.params.slowma)
        self.FastEMA = bt.indicators.EMA(
            self.datas[0], period=self.params.fastma)

        self.Trend.bollinger_touch = ""
        self.Trend.Status = ""
        self.Trend.Enter = ""
        self.Trend.ExitParameters = {
            "20BarRule": False, "EMARule": False, "OppositeBollingerTouch": False}
        self.Trend.EMARule = {"Close1": False, "Close2": False}
        self.Trend.Position = 0
        self.Trend.Test = 0

    def nextstart(self):
        self.ohlc.open = self.datas[0].open[0]
        self.ohlc.high = self.datas[0].high[0]
        self.ohlc.low = self.datas[0].low[0]
        self.ohlc.close = self.datas[0].close[0]

    def next(self):

        if self.Trend.bollinger_touch == "Top" and self.data.low[0] > self.BB.lines.bot[0] and self.Trend.Enter == "":
            # if the upper bollinger band was touched and the candle we have now makes the highest high and highest close,
            # we enter in an uptrend
            if self.data.high[0] > self.ohlc.high and self.data.close[0] > self.ohlc.close:
                self.Trend.Enter = "Uptrend entered"
                self.ohlc.high = self.data.high[0]
                self.Trend.Status = "Up"
                self.Trend.ExitParameters["OppositeBollingerTouch"] = False
                self.Trend.ExitParameters["20BarRule"] = False
                self.Trend.ExitParameters["EMARule"] = False
                self.Trend.Position = len(self.ohlc)
                self.Trend.Test = len(self.data.high)
            if self.data.high[0] > self.ohlc.high:
                self.ohlc.high = self.data.high[0]
            if self.data.close[0] > self.ohlc.close:
                self.ohlc.close = self.data.close[0]

        # we enter in a downtrend
        elif self.Trend.bollinger_touch == 'Bottom' and self.data.high[0] < self.BB.lines.top[0] and self.Trend.Enter == "":
            # if the bottom bollinger band was touched and the candle we have now makes the lowest low and lowest close,
            # we enter in a downtrend
            if self.data.low[0] < self.ohlc.low and self.data.close[0] < self.ohlc.close:
                self.Trend.Enter = "Downtrend entered"
                self.ohlc.low = self.data.low[0]
                self.Trend.Status = "Down"
                self.Trend.ExitParameters["OppositeBollingerTouch"] = False
                self.Trend.ExitParameters["20BarRule"] = False
                self.Trend.ExitParameters["EMARule"] = False
                self.Trend.Position = len(self.ohlc)
                self.Trend.Test = len(self.data.low)
            if self.data.low[0] < self.ohlc.low:
                self.ohlc.low = self.data.low[0]
            if self.data.close[0] < self.ohlc.close:
                self.ohlc.close = self.data.close[0]

        elif self.Trend.bollinger_touch == "":
            # look for a top bollinger touch, then store that bar's data
            if self.data.high[0] >= self.BB.lines.top[0] and self.data.low[0] > self.BB.lines.bot[0]:
                self.ohlc.open = self.data.open[0]
                self.ohlc.high = self.data.high[0]
                self.ohlc.low = self.data.low[0]
                self.ohlc.close = self.data.close[0]
                self.Trend.bollinger_touch = "Top"

            elif self.data.low[0] <= self.BB.lines.bot[0] and self.data.high[0] < self.BB.lines.top[0]:
                self.ohlc.open = self.data.open[0]
                self.ohlc.high = self.data.high[0]
                self.ohlc.low = self.data.low[0]
                self.ohlc.close = self.data.close[0]
                self.Trend.bollinger_touch = "Bottom"

        # if the upper bollinger band was touched but now the lower bollinger band got touched, the upper bollinger touch resets
        if self.Trend.bollinger_touch == "Top" and self.data.low[0] <= self.BB.lines.bot[0]:
                self.Trend.bollinger_touch = "Bottom"
                self.ohlc.open = self.data.open[0]
                self.ohlc.high = self.data.high[0]
                self.ohlc.low = self.data.low[0]
                self.ohlc.close = self.data.close[0]

        # if the bottom bollinger band was touched but now the top bollinger band got touched, the bottom bollinger touch resets
        elif self.Trend.bollinger_touch == "Bottom" and self.data.high[0] >= self.BB.lines.top[0]:
                self.Trend.bollinger_touch = "Top"
                self.ohlc.open = self.data.open[0]
                self.ohlc.high = self.data.high[0]
                self.ohlc.low = self.data.low[0]
                self.ohlc.close = self.data.close[0]

        # we are in an uptrend
        if self.Trend.Status == "Up":
            # if the opposite bollinger band was touched, we exit the uptrend
            if self.data.low[0] <= self.BB.lines.bot[0]:
                self.Trend.Status = ""
                self.Trend.Enter = ""
                self.Trend.ExitParameters["OppositeBollingerTouch"] = True
                self.Trend.bollinger_touch = "Bottom"
                self.Trend.EMARule['Close1'] = False
                self.Trend.EMARule['Close2'] = False
                self.ohlc.open = self.data.open[0]
                self.ohlc.high = self.data.high[0]
                self.ohlc.low = self.data.low[0]
                self.ohlc.close = self.data.close[0]
            # if a fresh high is made, keep track of the position
            if self.data.high[0] > self.ohlc.high:
                self.ohlc.high = self.data.high[0]
                self.Trend.Position = len(self.data.high)
            # after 20 bars (excluding the first) no new high is made, we are not in a trend anymore
            elif self.data.high[0] < self.ohlc.high:
                if len(self.data.high) - self.Trend.Position >= 21:
                    self.Trend.Status = ""
                    self.Trend.Enter = ""
                    self.Trend.ExitParameters["20BarRule"] = True
                    self.Trend.bollinger_touch = "Bottom"
                    self.Trend.EMARule['Close1'] = False
                    self.Trend.EMARule['Close2'] = False
                    self.Trend.ExitParameters['20BarRule'] = True

            # if the top bollinger band was touched, reset the EMA rule
            if self.data.high[0] >= self.BB.lines.top[0]:
                self.Trend.EMARule['Close1'] = False
                self.Trend.EMARule['Close2'] = False

            if self.data.close[0] < self.SlowEMA[0] and self.data.close[0] < self.FastEMA[0]:
                self.Trend.EMARule['Close1'] = True

            if self.Trend.EMARule['Close1'] == True and self.data.close[0] > self.SlowEMA[0] and self.data.close[0] > self.FastEMA[0]:
                self.Trend.EMARule['Close2'] = True

            if self.Trend.EMARule['Close1'] == True and self.Trend.EMARule['Close2'] == True and self.data.close[0] < self.SlowEMA[0] and self.data.close[0] < self.FastEMA[0]:
                self.Trend.EMARule['Close1'] = False
                self.Trend.EMARule['Close2'] = False
                self.Trend.Status = ""
                self.Trend.Enter = ""
                self.Trend.ExitParameters['EMARule'] = True
                self.Trend.bollinger_touch = ""

        if self.Trend.Enter == "Uptrend entered" and True not in self.Trend.ExitParameters.values():
            self.Trend[0] = 1

        # we are in a downtrend
        if self.Trend.Status == "Down":
            # if the opposite bollinger band was touched, we exit the downtrend
            if self.data.high[0] >= self.BB.lines.top[0]:
                self.Trend.Status = ""
                self.Trend.Enter = ""
                self.Trend.ExitParameters["OppositeBollingerTouch"] = True
                self.Trend.bollinger_touch = "Top"
                self.Trend.EMARule['Close1'] = False
                self.Trend.EMARule['Close2'] = False
                self.ohlc.open = self.data.open[0]
                self.ohlc.high = self.data.high[0]
                self.ohlc.low = self.data.low[0]
                self.ohlc.close = self.data.close[0]

            # if a fresh low is made, keep track of the position
            if self.data.low[0] < self.ohlc.high:
                self.ohlc.low = self.data.low[0]
                self.Trend.Position = len(self.data.low)
            # after 20 bars (excluding the first) no new low is made, we are not in a trend anymore
            elif self.data.low[0] > self.ohlc.low:
                if len(self.data.low) - self.Trend.Position >= 21:
                    self.Trend.Status = ""
                    self.Trend.Enter = ""
                    self.Trend.ExitParameters["20BarRule"] = True
                    self.Trend.bollinger_touch = ""
                    self.Trend.EMARule['Close1'] = False
                    self.Trend.EMARule['Close2'] = False
                    self.Trend.ExitParameters['20BarRule'] = True

            # if the bottom bollinger band was touched, reset the EMA rule
            if self.data.low[0] <= self.BB.lines.bot[0]:
                self.Trend.EMARule['Close1'] = False
                self.Trend.EMARule['Close2'] = False

            if self.data.close[0] > self.SlowEMA[0] and self.data.close[0] > self.FastEMA[0]:
                self.Trend.EMARule['Close1'] = True

            if self.Trend.EMARule['Close1'] == True and self.data.close[0] < self.SlowEMA[0] and self.data.close[0] < self.FastEMA[0]:
                self.Trend.EMARule['Close2'] = True

            if self.Trend.EMARule['Close1'] == True and self.Trend.EMARule['Close2'] == True and self.data.close[0] > self.SlowEMA[0] and self.data.close[0] > self.FastEMA[0]:
                self.Trend.EMARule['Close1'] = False
                self.Trend.EMARule['Close2'] = False
                self.Trend.Status = ""
                self.Trend.Enter = ""
                self.Trend.ExitParameters['EMARule'] = True
                self.Trend.bollinger_touch = ""

        if self.Trend.Enter == "Downtrend entered" and True not in self.Trend.ExitParameters.values():
            self.Trend[0] = -1

oh, somehow the way it will not work that way is gone. In init you will not be able to access data that way you do it.

Check out the Stage 1 - Stage 2 difference in this doc:
https://www.backtrader.com/docu/concepts/#stage-1-operators-create-objects
https://www.backtrader.com/docu/concepts/#stage-2-operators-true-to-nature

you have some issues with your code:

        self.ohlc.open = self.datas[0].open[0]
        self.ohlc.high = self.datas[0].high[0]
        self.ohlc.low = self.datas[0].low[0]
        self.ohlc.close = self.datas[0].close[0]

so change it to:

        self.ohlc.open = None
        self.ohlc.high = None
        self.ohlc.low = None
        self.ohlc.close = None

you also don't need to add a minperiod, since next will be called, when your indicators are filled.

if you want to init the attributes, you assign to the lines (?) ex. this way: self.Trend.Status = "", which works but is confusing, you could also set the inital ohlc values in nextstart:

def nextstart(self):
                self.ohlc.open = self.data.open[0]
                self.ohlc.high = self.data.high[0]
                self.ohlc.low = self.data.low[0]
                self.ohlc.close = self.data.close[0]

see: https://www.backtrader.com/docu/operating/

Provide some sample code to show the problem. Basically you try to access data which is not available. That’s all that can be said with the error you posted.

I mean ema. So in your case self.addminperiod(21) or higher if you do something with past ema data