Add missing dates to pandas dataframe

Question

My data can have multiple events on a given date or NO events on a date. I take these events, get a count by date and plot them. However, when I plot them, my two series don't always match.

idx = pd.date_range(df['simpleDate'].min(), df['simpleDate'].max())
s = df.groupby(['simpleDate']).size()

In the above code idx becomes a range of say 30 dates. 09-01-2013 to 09-30-2013 However S may only have 25 or 26 days because no events happened for a given date. I then get an AssertionError as the sizes dont match when I try to plot:

fig, ax = plt.subplots()    
ax.bar(idx.to_pydatetime(), s, color='green')

What's the proper way to tackle this? Do I want to remove dates with no values from IDX or (which I'd rather do) is add to the series the missing date with a count of 0. I'd rather have a full graph of 30 days with 0 values. If this approach is right, any suggestions on how to get started? Do I need some sort of dynamic reindex function?

Here's a snippet of S ( df.groupby(['simpleDate']).size() ), notice no entries for 04 and 05.

09-02-2013     2
09-03-2013    10
09-06-2013     5
09-07-2013     1

Answer 1

You could use Series.reindex:

import pandas as pd

idx = pd.date_range('09-01-2013', '09-30-2013')

s = pd.Series({'09-02-2013': 2,
               '09-03-2013': 10,
               '09-06-2013': 5,
               '09-07-2013': 1})
s.index = pd.DatetimeIndex(s.index)

s = s.reindex(idx, fill_value=0)
print(s)

yields

2013-09-01     0
2013-09-02     2
2013-09-03    10
2013-09-04     0
2013-09-05     0
2013-09-06     5
2013-09-07     1
2013-09-08     0
...