Ambiguity in Pandas Dataframe / Numpy Array "axis" definition

It's perhaps simplest to remember it as 0=down and 1=across.

This means:

• Use axis=0 to apply a method down each column, or to the row labels (the index).
• Use axis=1 to apply a method across each row, or to the column labels.

Here's a picture to show the parts of a DataFrame that each axis refers to:

It's also useful to remember that Pandas follows NumPy's use of the word axis. The usage is explained in NumPy's glossary of terms:

Axes are defined for arrays with more than one dimension. A 2-dimensional array has two corresponding axes: the first running vertically downwards across rows (axis 0), and the second running horizontally across columns (axis 1). [my emphasis]

So, concerning the method in the question, df.mean(axis=1), seems to be correctly defined. It takes the mean of entries horizontally across columns, that is, along each individual row. On the other hand, df.mean(axis=0) would be an operation acting vertically downwards across rows.

Similarly, df.drop(name, axis=1) refers to an action on column labels, because they intuitively go across the horizontal axis. Specifying axis=0 would make the method act on rows instead.

There are already proper answers, but I give you another example with > 2 dimensions.

The parameter axis means axis to be changed.
For example, consider that there is a dataframe with dimension a x b x c.

• df.mean(axis=1) returns a dataframe with dimenstion a x 1 x c.
• df.drop("col4", axis=1) returns a dataframe with dimension a x (b-1) x c.

Here, axis=1 means the second axis which is b, so b value will be changed in these examples.

Another way to explain:

// Not realistic but ideal for understanding the axis parameter df = pd.DataFrame([[1, 1, 1, 1], [2, 2, 2, 2], [3, 3, 3, 3]],                  columns=["idx1", "idx2", "idx3", "idx4"],                  index=["idx1", "idx2", "idx3"]                 )---------------------------------------1|          idx1  idx2  idx3  idx4|    idx1     1     1     1     1|    idx2     2     2     2     2|    idx3     3     3     3     30

About df.drop (axis means the position)

A: I wanna remove idx3.B: **Which one**? // typing while waiting response: df.drop("idx3",A: The one which is on axis 1B: OK then it is >> df.drop("idx3", axis=1)// Result---------------------------------------1|          idx1  idx2     idx4|    idx1     1     1     1|    idx2     2     2     2|    idx3     3     3     30

About df.apply (axis means direction)

A: I wanna apply sum.B: Which direction? // typing while waiting response: df.apply(lambda x: x.sum(),A: The one which is on *parallel to axis 0*B: OK then it is >> df.apply(lambda x: x.sum(), axis=0)// Resultidx1    6idx2    6idx3    6idx4    6