[Tutor] difference between expressions and statements

[bob gailer]

Caveat: I began this before there were any other responses. So this may 
be overkill - but I ike to be thorough.

On 4/9/2014 12:49 PM, Jared Nielsen wrote:
> Hi Pythons,
> Could someone explain the difference between expressions and statements?
>> I know that expressions are statements that produce a value.
> No. Expressions are not statements. These are mutually exclusive. 
> Expressions do produce values.
An attempt at a thorough answer:

In the language reference glossary under "expression" you will find:

"A piece of syntax which can be evaluated to some value. In other words, 
an expression is an accumulation of expression elements like literals, 
names, attribute access, operators or function calls which all return a 
value.... There are also statements which cannot be used as expressions, 
such as if. Assignments are also statements, not expressions."

Tthe above is a quote; I don't like some of the grammar.

In your examples print is a function. So all calls to print are expressions.

In the language reference you will also find:

7. Simple statements
7.1. Expression statements
7.2. Assignment statements
7.3. The assert statement
7.4. The pass statement
7.5. The del statement
7.6. The return statement
7.7. The yield statement
7.8. The raise statement
7.9. The break statement
7.10. The continue statement
7.11. The import statement
7.12. The global statement
7.13. The nonlocal statement
8. Compound statements
8.1. The if statement
8.2. The while statement
8.3. The for statement
8.4. The try statement
8.5. The with statement
8.6. Function definitions
8.7. Class definitions

With the exception of
- 7.1. Expression statements
- all of the above are either start with a keyword except 7.2 assignment 
which is indicated by an equal sign (=) .
- all of the above cause something to happen (except pass), and do not 
return a value.

7.1. Expression statement is either one expression or several separated 
by commas.
Used interactively to display value(s).
Used anywhere to make a function call.
> I'm unclear on functions and especially strings.
> Are any of the following expressions?
>
> print(42)
> print("spam")
> spam = 42
> print(spam)
>
> Is the first example producing a value or simply displaying an integer?
All function calls return a value. In the case of print the return value 
is always None.
spam = 42 is a statement. (indicated by the = sign. 42 is a value.
> Does a string count as a value?
> Yes - however I suspect you are limiting "string" to something within quotes. Those are "string literals".
> Is a variable assignment considered a value?
No
> If I print a variable is that considered production of a value?
See above comment on print.

Long but comprehensive answer. Feel free to ask questions.

Note there are various subtleties here -some  keywords may be used to 
start a statement or in an expression - e.g. if, else, for yield.

This also raises the fact that else (inter ala) is neither an expression 
or a statement; rather it is part of a compound statement. Nothing is 
simple.

Oh there is more but I may never hit send....