In Learning Python (a must read for anyone interested in Python!), author Mark Lutz refers to statements and expressions frequently, but give a clear explanation of the two.
Rather than reinventing the wheel by providing my own definition and examples of the two, I found this Quora Q&A does that perfectly:
A statement is a complete line of code that performs some action, while an expression is any section of the code that evaluates to a value. Expressions can be combined “horizontally” into larger expressions using operators, while statements can only be combined “vertically” by writing one after another, or with block constructs. Every expression can be used as a statement (whose effect is to evaluate the expression and ignore the resulting value), but most statements cannot be used as expressions. Here’s a complete list of statements.
That print is a statement is one of many warts in the Python 2 language that’s corrected in Python 3. Now
Here’s a general rule of thumb: If you can print it, or assign it to a variable, it’s an expression. If you can’t, it’s a statement.
Here are some examples of expressions:
2 + 2 3 * 7 1 + 2 + 3 * (8 ** 9) - sqrt(4.0) min(2, 22) max(3, 94) round(81.5) "foo" "bar" "foo" + "bar" None True False 2 3 4.0
All of the above can be printed or assigned to a variable.
Here are some examples of statements:
if CONDITION: elif CONDITION: else: for VARIABLE in SEQUENCE: while CONDITION: try: except EXCEPTION as e: class MYCLASS: def MYFUNCTION(): return SOMETHING yield SOMETHING raise SOMETHING with SOMETHING:
None of the above constructs can be assigned to a variable. They are syntactic elements that serve a purpose, but do not themselves have any intrinsic “value”. In other words, these constructs don’t “evaluate” to anything. Trying to do any of the following, for example, would be absurd, and simply wouldn’t work:
x = if CONDITION: y = while CONDITION: z = return 42 foo = for i in range(10):
Now as for your mentioning of print in Python… it varies. In Python 2, print is a statement, which means it is a single command that does not “evaluate” to anything, and therefore cannot be assigned to a variable. However, in Python 3, print is a function, which is a specific kind of expression.
“But wait!” you object. “If print is an expression, how come we can’t assign it to a variable?”
As it turns out, in Python 3 (but not Python 2) you can assign it to a variable:
x = print(42) print("The value of x is:") print(x)
If you run this code, you get the following:
42 'The value of x is': None
Aha! As it turns out, the print function in Python 3 does have a value. Specifically, its value is None, just like any other function that doesn’t specifically return anything:
def foo(): return # This function doesn't return anything. x = foo() print(x) # This will print `None`.
As always, if you have any questions or have a topic that you would like me to discuss, please feel free to post a comment at the bottom of this blog entry, e-mail at firstname.lastname@example.org, or drop me a message on Reddit (OzNetNerd).
Note: The opinions expressed in this blog are my own and not those of my employer.