for size in [1, 2, 3, 4]:result = 0print("size=" + str(size))for element in range(size):result = result + elementprint(" adding " + str(element)+", result so far=" + str(result))print("Done. size=" + str(size) + " result=" + str(result))
print("All done!")

I wonder why we use the str() function on size and result in the 7th line. I know we can't use integer and string in a single print function, but if the rule is like this, why isn't there any problem in this code (about integer + string rule)?

x=18
print("Hello , I am" , x , "years old.")

We use integer and string in a single line don't we ?

It's about the order things are run:

"Done. size=" + str(size) + " result=" + str(result)

has to be done first, before the print function is called. The print function just gets the single joined string - it has no idea that a + was used to construct it.

In this case:

print("Hello , I am" , x , "years old.")

the print function gets all 4 arguments (actually, a 4 element tuple) and can now work its magic on converting each one to a string itself (see Unnecessary detail below).

By the way, if you object to having to use str() then there are other ways of formatting the string to print. The following lines all produce the same output:

print("Done. size=" + str(size) + " result=" + str(result))
print("Done. size=%d result=%d" % (size, result))
print("Done. size={} result={}".format(size, result))
print(f"Done. size={size:} result={result:}")

The last one f" " requires Python 3.6 or later.

Unnecessary detail:

The conversion to str() in the print() function is actually done (in the C implementation) by the python API call PyFile_WriteObject() using the flag Py_PRINT_RAW. If Py_PRINT_RAW is given, the str() of the object is written.