7. Compound statements
Compound statements contain (groups of) other statements; they affect or control the
execution of those other statements in some way. In general, compound statements span
multiple lines, although in simple incarnations a whole compound statement may be
contained in one line.
The if, while and for
statements implement traditional control flow constructs. try
specifies exception handlers and/or cleanup code for a group of statements. Function and
class definitions are also syntactically compound statements.
Compound statements consist of one or more `clauses.' A clause consists of a header and
a `suite.' The clause headers of a particular compound statement are all at the same
indentation level. Each clause header begins with a uniquely identifying keyword and ends
with a colon. A suite is a group of statements controlled by a clause. A suite can be one
or more semicolon-separated simple statements on the same line as the header, following
the header's colon, or it can be one or more indented statements on subsequent lines. Only
the latter form of suite can contain nested compound statements; the following is illegal,
mostly because it wouldn't be clear to which if clause a
following else clause would belong:
if test1: if test2: print x
Also note that the semicolon binds tighter than the colon in this context, so that in
the following example, either all or none of the print statements
are executed:
if x < y < z: print x; print y; print z
Summarizing:
-
Note that statements always end in a NEWLINE
possibly followed by a DEDENT.
Also note that optional continuation clauses always begin with a keyword that cannot
start a statement, thus there are no ambiguities (the `dangling else'
problem is solved in Python by requiring nested if statements to
be indented).
The formatting of the grammar rules in the following sections places each clause on a
separate line for clarity.
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