系列正则表达式提取生成数据帧

Question

I am working through a regex task on Dataquest. The following code snippet runs correctly inside of the Dataquest IDE:

titles = hn["title"]
pattern = r'\[(\w+)\]'
tag_matches = titles.str.extract(pattern)
tag_freq = tag_matches.value_counts()
print(tag_freq, '\n')

However, on my PC running pandas 0.25.3 this exact same code block yields an error:

Traceback (most recent call last):
  File "C:/Users/Mark/PycharmProjects/main/main.py", line 63, in <module>
    tag_freq = tag_matches.value_counts()
  File "C:\Users\Mark\AppData\Local\Programs\Python\Python37-32\lib\site-packages\pandas\core\generic.py", line 5179, in __getattr__
    return object.__getattribute__(self, name)
AttributeError: 'DataFrame' object has no attribute 'value_counts' 

Why is tag_matches coming back as a dataframe? I am running an extract against the series 'titles'.

Answer 1

From the docs: Pandas.Series.str.Extract

A pattern with one group will return a Series if expand=False.

    >>> s.str.extract(r'[ab](\d)', expand=False)
0      1
1      2
2    NaN
dtype: object

So perhaps you must be explicit and set expand=False to get a series object?