5. Columns

We’ll begin with the latimes_make_and_model column, which records the standardized name of each helicopter that crashed. To access its contents separate from the rest of the DataFrame, append a period to the variable followed by the column’s name.

Hide code cell content
import pandas as pd
accident_list = pd.read_csv("https://raw.githubusercontent.com/palewire/first-python-notebook/main/docs/src/_static/ntsb-accidents.csv")
accident_list.latimes_make_and_model
0           BELL 407
1       ROBINSON R22
2       ROBINSON R44
3       ROBINSON R44
4       ROBINSON R44
           ...      
158         BELL 407
159    SCHWEIZER 269
160         BELL 206
161       AIRBUS 350
162     ROBINSON R44
Name: latimes_make_and_model, Length: 163, dtype: object

That will list the column out as a Series, just like the ones we created from scratch earlier. Just as we did then, you can now start tacking on additional methods that will analyze the contents of the column.

Note

You can also access columns a second way, like this: accident_list['latimes_make_and_model']. This method isn’t as pretty, but it’s required if your column has a space in its name, which would break the simpler dot-based method.

5.1. Count a column’s values

In this case, the column is filled with characters. So we don’t want to calculate statistics like the median and average, as we did before.

There’s another built-in pandas tool that will total up the frequency of values in a column. The method is called value_counts and it’s just as easy to use as sum, min or max. All you need to do it is add a period after the column name and chain it on the tail end of your cell.

accident_list.latimes_make_and_model.value_counts()
ROBINSON R44             38
BELL 206                 30
AIRBUS 350               29
ROBINSON R22             20
BELL 407                 13
HUGHES 369               13
MCDONNELL DOUGLAS 369     6
SCHWEIZER 269             5
AIRBUS 135                4
SIKORSKY 76               2
AGUSTA 109                2
AIRBUS 130                1
Name: latimes_make_and_model, dtype: int64

Congratulations, you’ve made your first finding. With that little line of code, you’ve calculated an important fact: During the period being studied, the Robinson R44 had more fatal accidents than any other helicopter.

5.2. Reset a DataFrame

You may notice that even though the result has two columns, pandas did not return a clean-looking table in the same way as head did for our DataFrame. That’s because our column, a Series, acts a little bit different than the DataFrame created by read_csv. In most instances, you can convert ugly Series into a pretty DataFrame by tacking on the reset_index method on the end.

accident_list.latimes_make_and_model.value_counts().reset_index()
index latimes_make_and_model
0 ROBINSON R44 38
1 BELL 206 30
2 AIRBUS 350 29
3 ROBINSON R22 20
4 BELL 407 13
5 HUGHES 369 13
6 MCDONNELL DOUGLAS 369 6
7 SCHWEIZER 269 5
8 AIRBUS 135 4
9 SIKORSKY 76 2
10 AGUSTA 109 2
11 AIRBUS 130 1

Why does a Series behave differently than a DataFrame? Why does reset_index have such a weird name?

Like so much in computer programming, the answer is simply, “because the people who created the library said so.” It’s important to learn that all open-source programming tools are made by humans, and humans have their quirks. Over time you’ll see pandas has more than a few.

As a beginner, you should just accept the oddities and keep moving. As you get more advanced, if there’s something about the system you think could be improved you should consider contributing to the Python code that operates the library.

Before we move on to the next chapter, here’s a challenge. See if you can answer a few more questions a journalist might ask about our dataset. All four of the questions below can be answered using only tricks we’ve covered thus far. See if you can do it.

  1. What was the total number of fatalities?

  2. Which helicopter maker had the most accidents?

  3. What was the total number of helicopter accidents by year?

  4. Which state had the most helicopter accidents?

Once you’ve written code answered those, you’re ready to move on to the next chapter.