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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 | |
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| 180 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> |
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