ValueError: Wrong number of items passed 500, placement implies 1, Python and Pandas

Question

I'm importing just two columns from .xlsx file and I would like to calculate some stuff (mean, deviation, percent change) and then I would like to plot all this. First part doesn't give me any problems, but plotting does.

My code looks like this:

import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
import matplotlib.mlab as mlab
import math

df = pd.read_excel('KDPrviIzbor.xlsx', sheetname='List1', index_col = 0)
ch = df.pct_change(periods=252)

ma = np.mean(ch)*100
std = np.std(ch)*100

x = np.linspace(-100,100,500)
plt.plot(x,mlab.normpdf(x,ma,std))

plt.show()

But when I run my code, I get this error:

Traceback (most recent call last):
File "C:/Users/David/PythonStuff/normal_distribution.py", line 21, in <module> plt.plot(x,mlab.normpdf(x,ma,std))
File "C:\Python27\lib\site-packages\matplotlib\mlab.py", line 1579, in normpdf return 1./(np.sqrt(2*np.pi)*sigma)*np.exp(-0.5 * (1./sigma*(x - mu))**2)
File "C:\Python27\lib\site-packages\pandas\core\ops.py", line 534, in wrapper dtype=dtype)
File "C:\Python27\lib\site-packages\pandas\core\series.py", line 220, in __init__ data = SingleBlockManager(data, index, fastpath=True)
File "C:\Python27\lib\site-packages\pandas\core\internals.py", line 3383, in __init__ ndim=1, fastpath=True)
File "C:\Python27\lib\site-packages\pandas\core\internals.py", line 2101, in make_block placement=placement)
File "C:\Python27\lib\site-packages\pandas\core\internals.py", line 77, in __init__ len(self.values), len(self.mgr_locs)))
ValueError: Wrong number of items passed 500, placement implies 1`

I figured that the problem is in:

plt.plot(x,mlab.normpdf(x,ma,std))

but I cannot solve it. Any suggestions?

Solution

ma and std are pandas.Series objects in your example. The reason is, that np.mean applied to a pandas.DataFrame returns a pandas.Series. However, mlab.normpdf(x,ma,std) expects float values or numpy arrays as inputs. You could simply convert ma and std to floats by ma = float(ma). I would not suggest to use int(ma) as you pointed out in your comment, because that would cut away the decimals.

Comments

[DavidV]

Fabian, thanks again. I used float(ma) and float(std) and now works as it should.