EBNF: Some Python constructs

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One can learn a lot about a programming language by inspecting the grammar and investigating parts of the language that one may never have encountered before.

The official Python EBNF grammar is at https://docs.python.org/3/reference/grammar.html .

There may be parts of this grammar that you have never encountered.

Studying the grammar of a language can help discover parts of the language that might be useful. Or that one might find used in examples on the Internet. Here are some EBNF rules for the grammar of the Python programming language, reformatted some from the official on-line grammar, that might be interesting.

Here is the associated EBNF (Extended Backus Naur Form) syntax diagram(s).

Syntax diagram for EBNF

Here is the associated EBNF grammar.

if_stmt = "if" then_part { elif_part } [ else_part ] . then_part = namedexpr_test ":" suite . elif_part = "elif" namedexpr_test ":" suite . else_part = "else" ":" suite .

Here is the associated EBNF syntax diagram(s).

Syntax diagram for EBNF

Here is the associated EBNF grammar.

while_stmt = "while" namedexpr_test ":" suite [ "else" ":" suite ] .

Did you know the Python while loop has an optional else part?

The else part of a Python while statement executes once whenever the while loop condition is or becomes false. It does not execute on a break out of the loop or if an exception is raised.

One use of this is to make it easier to break out of multiple levels of loops.

On the other hand, features that are not easily portable to other programming language may need to be used with some caution. Here is the Python code.

n1 = 4
print("while with else without break:")
i1 = 0
while i1 != n1:
	i1 += 1
	print("\ti1=[{0:d}]".format(i1))
else:
	print("\telse , i1=[{0:d}]".format(i1))
print("while with else with break:")
i1 = 0
while i1 != n1:
	i1 += 1
	print("\ti1=[{0:d}]".format(i1))
	if i1 == 2:
		break
else:
	print("\telse , i1=[{0:d}]".format(i1))
print("done")

Here is the output of the Python code.

while with else without break:
	i1=[1]
	i1=[2]
	i1=[3]
	i1=[4]
	else , i1=[4]
while with else with break:
	i1=[1]
	i1=[2]
done

Here is an example that shows how one might break outside a nested loop from within the inner loop using a while-else.

Is a clear way to do this? Here is the Python code.

m1 = 3
i1 = 0
print("nested loop:")
while i1 != m1:
	i1 += 1
	j1 = 0
	while j1 != m1:
		j1 += 1
		print("\ti1=[{0:d}] j1=[{1:d}]".format(i1,j1))
		if j1 == 2:
			break
	else:
		continue
	break
else:
	print("\telse , i1=[{0:d}]".format(i1))
print("done")

Here is the output of the Python code.

nested loop:
	i1=[1] j1=[1]
	i1=[1] j1=[2]
done

Here is the associated EBNF syntax diagram(s).

Syntax diagram for EBNF

Here is the associated EBNF grammar.

for_stmt = "for" exprlist "in" testlist ":" [ TYPE_COMMENT ] suite [ "else" ":" suite ] .

Did you know the Python for loop has an optional else part? It is similar in purpose as in the while loop.

And what exactly is a TYPE_COMMENT?