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<H2><A NAME="SECTION004180000000000000000"></A><A NAME="indentation"></A><a id='l2h-9' xml:id='l2h-9'></a>
<BR>
2.1.8 Indentation
</H2>

<P>
Leading whitespace (spaces and tabs) at the beginning of a logical
line is used to compute the indentation level of the line, which in
turn is used to determine the grouping of statements.

<P>
First, tabs are replaced (from left to right) by one to eight spaces
such that the total number of characters up to and including the
replacement is a multiple of
eight (this is intended to be the same rule as used by <span class="Unix">Unix</span>).  The
total number of spaces preceding the first non-blank character then
determines the line's indentation.  Indentation cannot be split over
multiple physical lines using backslashes; the whitespace up to the
first backslash determines the indentation.

<P>
<strong>Cross-platform compatibility note:</strong> because of the nature of
text editors on non-UNIX platforms, it is unwise to use a mixture of
spaces and tabs for the indentation in a single source file.  It
should also be noted that different platforms may explicitly limit the
maximum indentation level.

<P>
A formfeed character may be present at the start of the line; it will
be ignored for the indentation calculations above.  Formfeed
characters occurring elsewhere in the leading whitespace have an
undefined effect (for instance, they may reset the space count to
zero).

<P>
The indentation levels of consecutive lines are used to generate
INDENT and DEDENT tokens, using a stack, as follows.

<P>
Before the first line of the file is read, a single zero is pushed on
the stack; this will never be popped off again.  The numbers pushed on
the stack will always be strictly increasing from bottom to top.  At
the beginning of each logical line, the line's indentation level is
compared to the top of the stack.  If it is equal, nothing happens.
If it is larger, it is pushed on the stack, and one INDENT token is
generated.  If it is smaller, it <em>must</em> be one of the numbers
occurring on the stack; all numbers on the stack that are larger are
popped off, and for each number popped off a DEDENT token is
generated.  At the end of the file, a DEDENT token is generated for
each number remaining on the stack that is larger than zero.

<P>
Here is an example of a correctly (though confusingly) indented piece
of Python code:

<P>
<div class="verbatim"><pre>
def perm(l):
        # Compute the list of all permutations of l
    if len(l) &lt;= 1:
                  return [l]
    r = []
    for i in range(len(l)):
             s = l[:i] + l[i+1:]
             p = perm(s)
             for x in p:
              r.append(l[i:i+1] + x)
    return r
</pre></div>

<P>
The following example shows various indentation errors:

<P>
<div class="verbatim"><pre>
 def perm(l):                       # error: first line indented
for i in range(len(l)):             # error: not indented
    s = l[:i] + l[i+1:]
        p = perm(l[:i] + l[i+1:])   # error: unexpected indent
        for x in p:
                r.append(l[i:i+1] + x)
            return r                # error: inconsistent dedent
</pre></div>

<P>
(Actually, the first three errors are detected by the parser; only the
last error is found by the lexical analyzer -- the indentation of
<code>return r</code> does not match a level popped off the stack.)

<P>

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	55	<H2><A NAME="SECTION004180000000000000000"></A><A NAME="indentation"></A><a id='l2h-9' xml:id='l2h-9'></a>
	56	<BR>
	57	2.1.8 Indentation
	58	</H2>
	59
	60	<P>
	61	Leading whitespace (spaces and tabs) at the beginning of a logical
	62	line is used to compute the indentation level of the line, which in
	63	turn is used to determine the grouping of statements.
	64
65	<P>
66	First, tabs are replaced (from left to right) by one to eight spaces
67	such that the total number of characters up to and including the
68	replacement is a multiple of
69	eight (this is intended to be the same rule as used by <span class="Unix">Unix</span>). The
70	total number of spaces preceding the first non-blank character then
71	determines the line's indentation. Indentation cannot be split over
72	multiple physical lines using backslashes; the whitespace up to the
73	first backslash determines the indentation.
74
75	<P>
76	<strong>Cross-platform compatibility note:</strong> because of the nature of
77	text editors on non-UNIX platforms, it is unwise to use a mixture of
78	spaces and tabs for the indentation in a single source file. It
79	should also be noted that different platforms may explicitly limit the
80	maximum indentation level.
81
82	<P>
83	A formfeed character may be present at the start of the line; it will
84	be ignored for the indentation calculations above. Formfeed
85	characters occurring elsewhere in the leading whitespace have an
86	undefined effect (for instance, they may reset the space count to
87	zero).
88
89	<P>
90	The indentation levels of consecutive lines are used to generate
91	INDENT and DEDENT tokens, using a stack, as follows.
92
93	<P>
94	Before the first line of the file is read, a single zero is pushed on
95	the stack; this will never be popped off again. The numbers pushed on
96	the stack will always be strictly increasing from bottom to top. At
97	the beginning of each logical line, the line's indentation level is
98	compared to the top of the stack. If it is equal, nothing happens.
99	If it is larger, it is pushed on the stack, and one INDENT token is
100	generated. If it is smaller, it <em>must</em> be one of the numbers
101	occurring on the stack; all numbers on the stack that are larger are
102	popped off, and for each number popped off a DEDENT token is
103	generated. At the end of the file, a DEDENT token is generated for
104	each number remaining on the stack that is larger than zero.
105
106	<P>
107	Here is an example of a correctly (though confusingly) indented piece
108	of Python code:
109
110	<P>
111	<div class="verbatim"><pre>
112	def perm(l):
113	# Compute the list of all permutations of l
114	if len(l) <= 1:
115	return [l]
116	r = []
117	for i in range(len(l)):
118	s = l[:i] + l[i+1:]
119	p = perm(s)
120	for x in p:
121	r.append(l[i:i+1] + x)
122	return r
123	</pre></div>
124
125	<P>
126	The following example shows various indentation errors:
127
128	<P>
129	<div class="verbatim"><pre>
130	def perm(l): # error: first line indented
131	for i in range(len(l)): # error: not indented
132	s = l[:i] + l[i+1:]
133	p = perm(l[:i] + l[i+1:]) # error: unexpected indent
134	for x in p:
135	r.append(l[i:i+1] + x)
136	return r # error: inconsistent dedent
137	</pre></div>
138
139	<P>
140	(Actually, the first three errors are detected by the parser; only the
141	last error is found by the lexical analyzer -- the indentation of
142	<code>return r</code> does not match a level popped off the stack.)
143
144	<P>
145
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