pandas number of observations

### first method ###
len(df)
### second method ###
len(df.index)
### third method ###
df.shape[0]

Here is what the above code is Doing:
1. len(df) returns the number of rows in the dataframe.
2. len(df.index) returns the number of rows in the index.
3. df.shape[0] returns the number of rows in the dataframe.