jupyter notebook prints output even when suppressed - matplotlib

Question

Why is it printing the bins from the histogram?
Shouldn't the semicolon suppress it?
In [1]
import numpy as np
import matplotlib.pyplot as plt
from IPython.core.interactiveshell import InteractiveShell
InteractiveShell.ast_node_interactivity = "all";
In [2]
%matplotlib inline
data ={'first':np.random.rand(100),
'second':np.random.rand(100)}
fig, axes = plt.subplots(2)
for idx, k in enumerate(data):
axes[idx].hist(data[k], bins=20);

Answer 1

You've set InteractiveShell.ast_node_interactivity = "all";, so you've set all nodes to have ast interactivity enabled. So you get the values of data = {..}
And ; works only for the last top level expression, axes[idx].hist(data[k], bins=20); is not a top level expression, as it is nested in the for, the last top level node is the for, which is a statement.
Simply add a last no-op statement, and end it with ;
%matplotlib inline
data ={'first':np.random.rand(100),
'second':np.random.rand(100)};
fig, axes = plt.subplots(2);
for idx, k in enumerate(data):
axes[idx].hist(data[k], bins=20)
pass; # or None; 0; "foo"; ...
And you won't have any outputs.
Use codetransformer %%ast magic to quickly see the ast of an expression.

Answer 2

If you read the documentation, you will see exactly what it returns - a three item tuple described below. You can display it in the notebook by placing a ? at the end of the call to the histogram. It looks like your InteractiveShell is making it display. Normally, yes a semicolon would suppress the output, although inside of a loop it would be unnecessary.
Returns
n : array or list of arrays
The values of the histogram bins. See normed and weights
for a description of the possible semantics. If input x is an
array, then this is an array of length nbins. If input is a
sequence arrays [data1, data2,..], then this is a list of
arrays with the values of the histograms for each of the arrays
in the same order.
bins : array
The edges of the bins. Length nbins + 1 (nbins left edges and right
edge of last bin). Always a single array even when multiple data
sets are passed in.
patches : list or list of lists
Silent list of individual patches used to create the histogram
or list of such list if multiple input datasets.