freeCodeCamp/curriculum/challenges/chinese/08-data-analysis-with-python/data-analysis-with-python-c.../data-cleaning-introduction.md

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---
id: 5e9a093a74c4063ca6f7c15d
challengeType: 11
videoId: ovYNhnltVxY
---
## Description
<section id='description'>
More resources:
- <a href="https://notebooks.ai/rmotr-curriculum/data-cleaning-rmotr-freecodecamp-fd76fa59" target='_blank'>Notebook</a>
</section>
## Tests
<section id='tests'>
````yml
question:
text: |
What will the following code print out?
```py
import pandas as pd
import numpy as np
s = pd.Series(['a', 3, np.nan, 1, np.nan])
print(s.notnull().sum())
```
answers:
- |
3
- |
```
0 True
1 True
2 False
3 True
4 False
dtype: bool
```
- |
```
0 False
1 False
2 True
3 False
4 True
dtype: bool
```
solution: 1
````
</section>