The functions described in this chapter may be used either with or without parentheses around their
arguments. (The syntax descriptions omit the parentheses.) If you use the parentheses, the simple (but
occasionally surprising) rule is this: if it looks like a function, it is a function, and precedence doesn't
matter. Otherwise it's a list operator or unary operator, and precedence does matter. And whitespace
between the function and its left parenthesis doesn't count--so you need to be careful sometimes:

print 1+2+3; # Prints 6.
print(1+2) + 3; # Prints 3.
print (1+2)+3; # Also prints 3!
print +(1+2)+3; # Prints 6.
print ((1+2)+3); # Prints 6.

Je to v jednej o'reilly-ovke vysvetlovane hned na zaciatku.