I have a simple dataframe df2 that consist of indices and one column of values. I want to fit this dataframe to a poisson distribution. Below is the code I am using:

import numpy as np
from scipy.optimize import curve_fit    
def poisson(k, lamb):
    return (lamb^k/ np.math.factorial(k)) * np.exp(-lamb)

params, cov =  curve_fit(poisson, np.array(bins.tolist()), data.flatten())

I get the following error:

TypeError: only size-1 arrays can be converted to Python scalars


1 Answer 1


I think the cause of the error is the np.math.factorial(k) function call, since curve_fit passes a numpy array as the first parameter to the poisson function, and if you try to run the code

np.math.factorial(np.array([1, 2, 3]))

You'll get the error

TypeError: only size-1 arrays can be converted to Python scalars

Try using scipy.special.factorial since it accepts a numpy array as input instead of only accepting scalers.

Thus, just change your poisson function to

def poisson(k, lamb):
    return (lamb**k/ scipy.special.factorial(k)) * np.exp(-lamb)

Hope this helps

EDIT: Also I fixed the ^ to ** since that's how you use the exponential operator in python.

  • $\begingroup$ Thanks for your comment. I actually tried factorial from scipy but it gave me the below error, that's why I switched numpy: "TypeError: ufunc 'bitwise_xor' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''" $\endgroup$
    – HaneenSu
    May 5, 2020 at 9:57
  • $\begingroup$ Oh sorry forgot the second error in your code. You use ^ for the exponent but you actually need to use ** for exponents in python. $\endgroup$
    – A Kareem
    May 5, 2020 at 9:58

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