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i used wavelet decomposition command in python using pywt library but it does not return any coefficients. my code is given below .

import numpy as np
import pywt as pywt
(e,f)=pywt.wavedec(y,'db12' ,level=2)
print("e:"+str(e))
print("f:"+str(f))
I also tried with pywt.dwt(y,' db12', level=2) it is also not returning any coefficients
it returns a null output, where y is a matrix contains my input 
I tried reproducing your results with a random (discrete) signal like so:
import numpy as np
import pyw
x = np.random.randint(0,100,500)
y = pywt.wavedec(x, 'db12', level=2)
(e,f) = pywt.dwt(x, 'db12')
I noticed two things: For a 1D signal, wavedec returns more than two coefficient arrays, as also mentioned in the docs. Similarly, the dwt function does not know the keyword level=, but works just fine with the command specified above. 

Hope that helps
        
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