Sometime formal programming documentation is less than clear. At least, it’s less than clear until you’ve written your first solution. The Modules section of the Python language is one of those that takes a few moments to digest.
Chapters 22 and 23 in Learning Python gives some additional details but not a clear step-by-step approach to implementing Python modules. This post is designed to present the steps to write, import, and call a Python module. I figured that it would be helpful to write one for my students, and posting it in the blog seemed like the best idea.
I wrote the module to parse an Oracle version string into what we’d commonly expect to see, like the release number, an “R”, a release version, and then the full version number. The module name is more or less equivalent to a package name, and the file name is effectively the module name. The file name is
strVersionOracle.py, which makes the
strVersionOracle the module name.
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# Parse and format Oracle version. def formatVersion(s): # Split string into collection. list = s.split(".") # Iterate through the result set. for i, l in enumerate(list): if i == 0 and list[i] == "11": label = str(l) + "g" elif i == 0 and list[i] == "12": label = label + str(l) + "c" elif i == 1: label = label + "R" + list[i] + " (" + s + ")" # Return the formatted string. return label
You can put this in any directory as long as you add it to the Python path. There are two Python paths to maintain. One is in the file system and the other is in Python’s interactive IDLE environment. You can check the contents of the IDLE path with the following interactive commands:
import sys print sys.path
It prints the following:
['', '/usr/lib64/python27.zip', '/usr/lib64/python2.7', '/usr/lib64/python2.7/plat-linux2', '/usr/lib64/python2.7/lib-tk', '/usr/lib64/python2.7/lib-old', '/usr/lib64/python2.7/lib-dynload', '/usr/lib64/python2.7/site-packages', '/usr/lib64/python2.7/site-packages/gtk-2.0', '/usr/lib/python2.7/site-packages']
You can append to the IDLE path using the following command:
After putting the module in the runtime path, you can test the code in the IDLE environment:
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import cx_Oracle db = cx_Oracle.connect("student/student@xe") print strVersionOracle.formatVersion(db.version)
Line 3 prints the result by calling the formatVersion function inside the strVersionOracle module. It prints the following:
You can test the program outside of the runtime environment with the following
oracleConnection.py file. It runs
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# Import the Oracle library. import cx_Oracle import strVersionOracle try: # Create a connection. db = cx_Oracle.connect("student/student@xe") # Print a message. print "Connected to the Oracle " + strVersionOracle.formatVersion(db.version) + " database." except cx_Oracle.DatabaseError, e: error, = e.args print >> sys.stderr, "Oracle-Error-Code:", error.code print >> sys.stderr, "Oracle-Error-Message:", error.message finally: # Close connection. db.close()
You can call the
formatVersion() function rather than a combination of module and function names when you write a more qualified import statement on line 3, like:
from strVersionOracle import formatVersion
Then, you can call the
formatVersion() function like this on line 10:
print "Connected to the Oracle " + formatVersion(db.version) + " database."
It works because you told it to import a function from a Python module. The first example imports a module that may contain one to many functions, and that style requires you to qualify the location of functions inside imported modules.
oracleConnection.py program works when you call it from the Bash shell provided you do so from the same directory where the
strVersionOracle.py files (or Python modules) are located. If you call the
oracleConnection.py file from a different directory, the reference to the library raises the following error:
Traceback (most recent call last): File "oracleConnection.py", line 3, in <module> import strVersionOracle ImportError: No module named strVersionOracle
You can fix this error by adding the directory where the strVersionOracle.py file exists, like
export set PYTHONPATH=/home/student/Code/python
Then, you can call successfully the
oracleConnection.py file from any directory:
The program will connect to the Oracle database as the student user, and print the following message to the console:
Connected to the Oracle 11gR2 (188.8.131.52.0) database.
I hope this helps those trying to create and use Python modules.