Trend Step Channel - appreciate it about technical HOT 4 OPEN

a=0.,a := src > nz(a[1],src) + dev ? src : src < nz(a[1],src) - dev ? src : nz(a[1],src)
upper = valuewhen(change(a),a + dev,0)
lower = valuewhen(change(a),a - dev,0)

import pandas_ta as ta
## dfInd is the input dataframe that contain ohlcv data 

length = 100
fast = 50
slow = 200
src = dfInd['Close']  # control src : Close, hl2, hlc3, etc 

slowChange = ta.mom(src,length=length).abs()
fastChangeSum = ta.mom(src,length=1).abs().rolling(length).sum()

er = slowChange/fastChangeSum
dev = er*ta.stdev(src*2,fast) + (1-er)*ta.stdev(src*2,slow) # verified and correct 

## ------------- Code below this line is where i am struggling !! 

a = pd.DataFrame(0., index=dfInd.index, columns=['a'])
a=a['a']
# len(src.values)
# len(src.shift().fillna(0).values)

# # a['a'] = src.shift().fillna(0).values

value = np.where(a.shift(1).notnull(), a, src)

tsc = np.where (src > value + dev , value, src)
# tsc = np.where (src > value + dev , src, (np.where (src < value - dev , src , value)))  

dfInd['TSC'] = tsc
dfInd['TSCdev'] = dev

# ----------- not reached to upper and lower bands calculations yet
# chang_a = ta.mom(dfInd['TSC'],1)
# dfInd['TSC_U'] = dfInd['TSC'] + dev
# dfInd['TSC_L'] = dfInd['TSC'] - dev