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annotate Lib/whichdb.py @ 5817:48b94de85bf3 legacy-trunk

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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
parents b378753825a0
children fb4b4d069cc7
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c3ec69b1fe36 Function to guess which db package created a database.
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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 struct
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4
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5 def whichdb(filename):
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6 """Guess which db package to use to open a db file.
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7
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8 Return values:
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9
c3ec69b1fe36 Function to guess which db package created a database.
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10 - None if the database file can't be read;
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11 - empty string if the file can be read but can't be recognized
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12 - the module name (e.g. "dbm" or "gdbm") if recognized.
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13
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14 Importing the given module may still fail, and opening the
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15 database using that module may still fail.
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16 """
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17
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18 # Check for dbm first -- this has a .pag and a .dir file
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19 try:
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20 f = open(filename + ".pag", "rb")
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21 f.close()
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22 f = open(filename + ".dir", "rb")
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23 f.close()
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24 return "dbm"
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25 except IOError:
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26 pass
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27
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28 # See if the file exists, return None if not
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29 try:
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30 f = open(filename, "rb")
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31 except IOError:
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32 return None
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33
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34 # Read the first 4 bytes of the file -- the magic number
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35 s = f.read(4)
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36 f.close()
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37
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38 # Return "" if not at least 4 bytes
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39 if len(s) != 4:
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40 return ""
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41
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42 # Convert to 4-byte int in native byte order -- return "" if impossible
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43 try:
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44 (magic,) = struct.unpack("=l", s)
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45 except struct.error:
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46 return ""
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47
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48 # Check for GNU dbm
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49 if magic == 0x13579ace:
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50 return "gdbm"
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51
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52 # Check for BSD hash
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53 if magic == 0x061561:
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54 return "dbhash"
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55
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56 # Unknown
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57 return ""