Compute the histogram of a set of data.
Parameters: 

a : array_like 
Input data. The histogram is computed over the flattened array. 
bins : int or sequence of scalars or str, optional 
If bins is an int, it defines the number of equalwidth bins in the given range (10, by default). If bins is a sequence, it defines a monotonically increasing array of bin edges, including the rightmost edge, allowing for nonuniform bin widths. If bins is a string, it defines the method used to calculate the optimal bin width, as defined by histogram_bin_edges . 
range : (float, float), optional 
The lower and upper range of the bins. If not provided, range is simply (a.min(), a.max()) . Values outside the range are ignored. The first element of the range must be less than or equal to the second. range affects the automatic bin computation as well. While bin width is computed to be optimal based on the actual data within range , the bin count will fill the entire range including portions containing no data. 
normed : bool, optional 
Deprecated since version 1.6.0. This is equivalent to the density argument, but produces incorrect results for unequal bin widths. It should not be used. Changed in version 1.15.0: DeprecationWarnings are actually emitted. 
weights : array_like, optional 
An array of weights, of the same shape as a . Each value in a only contributes its associated weight towards the bin count (instead of 1). If density is True, the weights are normalized, so that the integral of the density over the range remains 1. 
density : bool, optional 
If False , the result will contain the number of samples in each bin. If True , the result is the value of the probability density function at the bin, normalized such that the integral over the range is 1. Note that the sum of the histogram values will not be equal to 1 unless bins of unity width are chosen; it is not a probability mass function. Overrides the normed keyword if given. 
Returns: 

hist : array 
The values of the histogram. See density and weights for a description of the possible semantics. 
bin_edges : array of dtype float 
Return the bin edges (length(hist)+1) . 
Notes
All but the last (righthandmost) bin is halfopen. In other words, if bins
is:
[1, 2, 3, 4]
then the first bin is [1, 2)
(including 1, but excluding 2) and the second [2, 3)
. The last bin, however, is [3, 4]
, which includes 4.
Examples
>>> np.histogram([1, 2, 1], bins=[0, 1, 2, 3])
(array([0, 2, 1]), array([0, 1, 2, 3]))
>>> np.histogram(np.arange(4), bins=np.arange(5), density=True)
(array([0.25, 0.25, 0.25, 0.25]), array([0, 1, 2, 3, 4]))
>>> np.histogram([[1, 2, 1], [1, 0, 1]], bins=[0,1,2,3])
(array([1, 4, 1]), array([0, 1, 2, 3]))
>>> a = np.arange(5)
>>> hist, bin_edges = np.histogram(a, density=True)
>>> hist
array([0.5, 0. , 0.5, 0. , 0. , 0.5, 0. , 0.5, 0. , 0.5])
>>> hist.sum()
2.4999999999999996
>>> np.sum(hist * np.diff(bin_edges))
1.0
Automated Bin Selection Methods example, using 2 peak random data with 2000 points:
>>> import matplotlib.pyplot as plt
>>> rng = np.random.RandomState(10) # deterministic random data
>>> a = np.hstack((rng.normal(size=1000),
... rng.normal(loc=5, scale=2, size=1000)))
>>> _ = plt.hist(a, bins='auto') # arguments are passed to np.histogram
>>> plt.title("Histogram with 'auto' bins")
Text(0.5, 1.0, "Histogram with 'auto' bins")
>>> plt.show()