在 Python 中打印元组列表会导致 [None,None,None]

Question

I have a list of Tuples that I want to print in Jupyter Notebooks. My code prints them correctly but seems to be printing a strange line at the end '[None,None,None]', I can't figure out why.

If I swap the list comprehension code with a for loop to print the same thing I don't get the '[None,None,None]' line. But I'm trying to write more easy to read code and would prefer to keep the list comprehension approach, there is definitely something I can learn from this mistake.

Check error message and code in the attached image

a = [(1, 108460.7476635514), (2, 103072.89682539682), (3, 77251.9265944645)]
[print('Group=', x[0], '; Avg=', x[1]) for x in a]

Answer 1

Jupyter notebooks return the value of the last variable in a code cell if there is no assignment operator (=) in the last statement. This code outputs the value of the list (the return value of all print statements, or None) because you're creating a list during list comprehension. Generally, it is bad practice to use list comprehension if you're not going to use the list it generates. The cleaner, more Pythonic way to do this is via a standard for loop:

a = [(1, 108460.7476635514), (2, 103072.89682539682), (3, 77251.9265944645)]
for i in a:
    print('Group=', x[0], '; Avg=', x[1])