[Tutor] Python "well-formed formulas"

[Steven D'Aprano]

On 28/05/13 21:44, Citizen Kant wrote:
> Could you please help me with a simple example of a Python well-formed
> formula in order to understand "well-formed formulas" and "formation rules"
> concepts properly?


Probably not, since they aren't really Python terms, so I'll be guessing what they are.

But *in general*, "well-formed" means there are no errors according to the grammar or syntax of the language being written. For example, in English:

"Last week, I went to the restaurant and ate a pizza."

is a grammatically-correct sentence, and so is well-formed. But:

"Last weeks, me goed to a restaurants and ated a pizzas?"

is not. Similarly, this is a well-formed mathematical expression:

y = 3x² - 4(x + 5)

while this is not:

y = ² - 4(x+)


In the case of Python, there are many different varieties of well-formed expressions. Some of them look remarkably close to mathematical notation (which is not an accident):

y = 3*x**2 - 4*(x + 5)

Some obvious differences: Python uses ** for the power operator, not superscript; multiplication must be explicitly stated using * and not just implied.

Other well-formed Python expressions:

mylist and mylist[1:]
"This is a string".upper()
function(12, 3.5, {4:None})

and some which are not well-formed:

mylist mylist:[1]
This is a string.)(upper
function 12,, 3.5, {4 None)}


Basically, something is well-formed if it has the form of the syntax or grammar of the language, and is not if it doesn't.




-- 
Steven