merge() missing 1 required positional argument: 'right'
<p>I am doing a tutorial and I have a problem:
<strong>My code:</strong></p>

<pre><code>import html5lib 
import quandl
import pandas as pd
import pickle

pd.read_html("https://simple.wikipedia.org/wiki/List_of_U.S._states")
main_df = pd.DataFrame()
fiddy_states = pd.read_html('https://simple.wikipedia.org/wiki/List_of_U.S._states')
for abbv in fiddy_states[0][0][1:]:
    query = "FMAC/HPI_"+str(abbv)
    df = quandl.get(query)

if main_df.empty:
    main_df = df 
else:
    main_df = pd.merge (main_df , df, how = "right")
print(pd.merge(main_df))
</code></pre>

<p><strong>and my error is</strong>: </p>

<pre><code>TypeError: merge() missing 1 required positional argument: 'right'
</code></pre>

<p>what's wrong? </p>

<pre><code>main_df.merge(df, how = "right")
</code></pre>

python pandas