I am learning `Pandas` package by replicating the outing from some of the R vignettes. Now I am using the `dplyr` package from R as an example:

<http://cran.rstudio.com/web/packages/dplyr/vignettes/introduction.html>

### R script

```
planes <- group_by(hflights_df, TailNum)
delay <- summarise(planes,
  count = n(),
  dist = mean(Distance, na.rm = TRUE))
delay <- filter(delay, count > 20, dist < 2000)

```

### Python script

```
planes = hflights.groupby('TailNum')
planes['Distance'].agg({'count' : 'count',
                        'dist' : 'mean'})

```

How can I state explicitly in python that `NA` needs to be skipped?