无法理解Python代码的结果

Question

I was learning about dictionaries in Python and I created a simple program:

# Create an empty dictionary called d1
d1 = {}

# Print dictionary and length
def dixnary():
    print "Dictionary contents : "
    print d1
    print "Length = ", len(d1)

# Add items to dictionary
d1["to"] = "two"
d1["for"] = "four"

print "Dictionary contents :"
print d1
print "Length =" , len(d1)

# Print dictionary and length
print dixnary()

Now there's a difference in the results when I use the print commands and when I use the dixnary function.

Using the print commands I get the result:

Dictionary contents:
<'to':'two','for:'four'>
Length = 2

and when I use the function dixnary , I get the results:

Dictionary contents:
<'to':'two','for:'four'>
Length = 2
None

Notice the None on the final line. This None gets added when I use the function dixnary . Why is this?

Answer 1

You're attempting to print the return value of a function, but the function doesn't return a value, so it returns the default value of None.

The reason why it prints out other data is that you have print commands inside of the function. Just run the function ( dixnary() ), instead of printing it ( print dixnary() ).

Or alternatively, have the program return the string, so you can do useful things with it.

def dixnary():
    return "Dictionary contents :\n%s\nLength = %d" % (d1, len(d1))

print dixnary()