Common mistakes in identifying missing values in Python

There are a few frequent mistakes that might happen while using Pandas to find missing values. Here are a few of them, with code examples:

1. Unrecognizing various missing value representations:

Missing values in Pandas can be represented by a number of different values, including NaN, None, or even bespoke values like "NA". Failure to take into account these various representations may result in incorrectly identifying missing values.

The empty string "" and the missing value "NA" in this example are not acknowledged as missing values. By combining the isnull() and isin() functions using the | operator (OR operator), we can correctly identify missing values in the DataFrame.

2. Unhandled missing values in numeric and non-numeric columns:

Missing values in columns that are either numeric or non-numeric (like strings) can be analyzed carefully before applying or calculating any value. Incorrect analyses or computations may result in incorrect outcomes.

In the first scenario, we try to use the mean() method to determine the mean of the "Numeric" column. But because the missing value is not handled, the mean calculation continues without taking it into account, giving an inaccurate mean value of nan rather than the anticipated 3.0.

In the second scenario, we use the str.len() function to estimate the length of each element in the 'NonNumeric' column. The output contains a missing value (NaN) instead of the anticipated length of the non-null values because the missing value is handled as a None or NaN throughout the calculation.

3. Unconsidered missing values in time series data:

It’s crucial to take into account missing values that result from gaps in the time index while working with time series data. Ignoring these missing variables could result in erroneous data handling or analysis.

Pandas did not infer the missing value in this example because of the time difference, therefore the missing value on 2023-01-03 is not detected. To correctly identify missing values while considering time gaps, we can use the pd.date_range() function to create a date range that spans from the minimum to the maximum value in the index. This ensures that all dates within that range are included, even if they are missing in the original DataFrame.

We then use data.reindex(date_range) to reindex the DataFrame with the complete date range. This fills any missing dates with NaN values.