Python : simple code gives "generator object", why? [duplicate]-CodePudding

I m trying to learn to code in a "pythonic way".

The original code is working, results and displays the expected results. The new code is giving: "<generator object at 0x0000017862039510>". Why then ?

Original code:

a={
    'AA':-5,
    'BB':-8,
    'C':15,
    'D':-85,
    'E':24
}

for i in a.values():
    if i<0:
        print(i)

New code :

a={
    'AA':-5,
    'BB':-8,
    'C':15,
    'D':-85,
    'E':24
}
        
print(i for i in a.values() if i<0 )

Thank you !

CodePudding user response：

i for i in a.values() if i<0 is a generator - it doesn't resolve to a sequence of values unless something iterates it. For parsing reasons, you can't have the generator by itself without enclosing it in grouping parenthesis

>>> i for i in a.values() if i<0
  File "<stdin>", line 1
    i for i in a.values() if i<0
      ^
SyntaxError: invalid syntax
>>> (i for i in a.values() if i<0)
<generator object <genexpr> at 0x7f2a54b3ac80>

print just takes the string representation of objects. It doesn't try to iterate them. So, you get the string representation of the generator itself. Other objects are different - lists and tuples for instance, do iterate their initialization value.

>>> list(i for i in a.values() if i<0)
[-5, -8, -85]
>>> tuple(i for i in a.values() if i<0)
(-5, -8, -85)

List comprehensions do the same thing - build lists using the same generator semantics

>>> [i for i in a.values() if i<0]
[-5, -8, -85]

CodePudding user response：

To print out the values of the generator, separated by a newline, you can use .join method which will create a string by joining each element of the generator with a newline (note that the values in the generator must be strings (in this case they need to be explicitly converted)):

print('\n'.join(str(i) for i in a.values() if i < 0))

CodePudding user response：

The expression which is written in that print statement of yours is a generator expression. In your original code, you iterate through each key-value pair in your dictionary and print the value. In your new code, you are attempting to print the result of that expression which is an object and not a series of print statements.