Floats print differently when shown in dictionary - python

Question

I'm creating a dictionary with this simple code:

pixel_histogram = {}
min_value = -0.2
max_value = 0.2
interval_size = (math.fabs(min_value) + math.fabs(max_value))/bins

for i in range(bins):
    key = min_value+(i*interval_size)
    print key
    pixel_histogram[key] = 0
    print pixel_histogram

But I'm a little surprised 'cause I got these values with my prints:

#Printing keys
-0.2
-0.16
-0.12
-0.08
-0.04
0.0
0.04
0.08
0.12
0.16

#Printing the dictionary
{0.0: 0, 
-0.08000000000000002: 0, 
0.15999999999999998: 0, 
-0.16: 0, 
0.12: 0, 
-0.12000000000000001: 0, 
0.08000000000000002: 0, 
-0.04000000000000001: 0, 
-0.2: 0, 
0.03999999999999998: 0}

I didn't figure out why the values are different and how could I solve this. Any help would be appreciated. Thanks.

Answer 1

Python's print statement uses str() on the item being printed. For floating-point values, str() will print up to certain number of decimal values.

When printing a dictionary, the print statement is calling str() on the dictionary object. The dictionary, in its __str__() method definition, uses repr() on the keys. The repr() function for floating-point values prints to more decimal places than the str() function does.

The reason dictionaries use repr() and not str() for keys is that you almost definitely want to see print {'1': 1} print differently than print {1: 1}.