title: Rolling ewma in pandas
body: <p>I'm trying to compute ewma in a rolling fashion, just like rolling mean, std etc... The code below does not work, I'm not sure what the issue is. </p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  
  a = pd.DataFrame(np.random.randn(50, 5))
  pd.rolling_apply(arg=a[0], 
                   func=lambda x: pd.ewma(x, span=10), 
                   window=10)
  </code></pre>
  
  <blockquote>
    <p>TypeError: only size-1 arrays can be converted to Python scalars</p>
  </blockquote>