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SF bug #690377: Dud footnote reference in distutils docs
author Raymond Hettinger <python@rcn.com>
date Fri, 21 Feb 2003 02:47:09 +0000
parents bcc35a0f1a78
children e107c921b8f3
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c3ec69b1fe36 Function to guess which db package created a database.
Guido van Rossum <guido@python.org>
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1 """Guess which db package to use to open a db file."""
c3ec69b1fe36 Function to guess which db package created a database.
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2
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3 import os
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4 import struct
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5
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6 try:
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7 import dbm
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8 _dbmerror = dbm.error
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9 except ImportError:
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10 dbm = None
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11 # just some sort of valid exception which might be raised in the
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12 # dbm test
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13 _dbmerror = IOError
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14
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15 def whichdb(filename):
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16 """Guess which db package to use to open a db file.
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17
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18 Return values:
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19
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20 - None if the database file can't be read;
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21 - empty string if the file can be read but can't be recognized
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22 - the module name (e.g. "dbm" or "gdbm") if recognized.
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23
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24 Importing the given module may still fail, and opening the
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25 database using that module may still fail.
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26 """
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27
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28 # Check for dbm first -- this has a .pag and a .dir file
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29 try:
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30 f = open(filename + os.extsep + "pag", "rb")
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31 f.close()
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32 f = open(filename + os.extsep + "dir", "rb")
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33 f.close()
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34 return "dbm"
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35 except IOError:
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36 # some dbm emulations based on Berkeley DB generate a .db file
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37 # some do not, but they should be caught by the dbhash checks
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38 try:
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39 f = open(filename + os.extsep + "db", "rb")
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40 f.close()
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41 # guarantee we can actually open the file using dbm
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42 # kind of overkill, but since we are dealing with emulations
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43 # it seems like a prudent step
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44 if dbm is not None:
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45 d = dbm.open(filename)
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46 d.close()
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47 return "dbm"
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48 except (IOError, _dbmerror):
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49 pass
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50
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51 # Check for dumbdbm next -- this has a .dir and and a .dat file
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52 try:
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53 f = open(filename + os.extsep + "dat", "rb")
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54 f.close()
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55 f = open(filename + os.extsep + "dir", "rb")
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56 try:
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57 if f.read(1) in ["'", '"']:
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58 return "dumbdbm"
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59 finally:
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60 f.close()
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61 except IOError:
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62 pass
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63
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64 # See if the file exists, return None if not
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65 try:
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66 f = open(filename, "rb")
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67 except IOError:
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68 return None
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69
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70 # Read the start of the file -- the magic number
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71 s16 = f.read(16)
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72 f.close()
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73 s = s16[0:4]
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74
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75 # Return "" if not at least 4 bytes
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76 if len(s) != 4:
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77 return ""
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78
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79 # Convert to 4-byte int in native byte order -- return "" if impossible
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80 try:
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81 (magic,) = struct.unpack("=l", s)
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82 except struct.error:
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83 return ""
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84
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85 # Check for GNU dbm
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86 if magic == 0x13579ace:
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87 return "gdbm"
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88
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89 # Check for BSD hash
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90 if magic in (0x00061561, 0x61150600):
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91 return "dbhash"
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92
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93 # BSD hash v2 has a 12-byte NULL pad in front of the file type
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94 try:
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95 (magic,) = struct.unpack("=l", s16[-4:])
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96 except struct.error:
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97 return ""
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98
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99 # Check for BSD hash
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100 if magic in (0x00061561, 0x61150600):
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101 return "dbhash"
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102
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103 # Unknown
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104 return ""