Error Handling Python 2.6
you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions. 8.1. Syntax Errors¶ Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python: >>> while True print('Hello world') File "
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end-user then by having them run into an issue with your software and displaying a big ugly error message on the screen, followed by a program crash. Exception handling is all about ensuring that when your program encounters an issue, it will continue to run and provide http://www.jython.org/jythonbook/en/1.0/ExceptionHandlingDebug.html informative feedback to the end-user or program administrator. Any Java programmer becomes familiar with exception handling on day one, as some Java code won’t even compile unless there is some form of exception handling put into place via the try-catch-finally syntax. http://sebastianraschka.com/Articles/2014_python_2_3_key_diff.html Python has similar constructs to that of Java, and we’ll discuss them in this chapter. After you have found an exception, or preferably before your software is distributed, you should go through the code and debug it in order to error handling find and repair the erroneous code. There are many different ways to debug and repair code; we will go through some debugging methodologies in this chapter. In Python as well as Java, the assert keyword can help out tremendously in this area. We’ll cover assert in depth here and learn the different ways that it can be used to help you out and save time debugging those hard-to-find errors. Exception Handling Syntax and Differences with Java¶ Java developers are very familiar with error handling python the try-catch-finally block as this is the main mechanism that is used to perform exception handling. Python exception handling differs a bit from Java, but the syntax is fairly similar. However, Java differs a bit in the way that an exception is thrown in code. Now, realize that I just used the term throw…this is Java terminology. Python does not throw exceptions, but instead it raises them. Two different terms which mean basically the same thing. In this section, we’ll step through the process of handling and raising exceptions in Python code, and show you how it differs from that in Java. For those who are unfamiliar, I will show you how to perform some exception handling in the Java language. This will give you an opportunity to compare the two syntaxes and appreciate the flexibility that Python offers. Listing 7-1. Exception Handling in Java try { // perform some tasks that may throw an exception } catch (ExceptionType messageVariable) { // perform some exception handling } finally { // execute code that must always be invoked } Now let’s go on to learn how to make this work in Python. Not only will we see how to handle and raise exceptions, but you’ll also learn some other great techniques such as using assertions later in the chapter. Catching Exceptions¶ How often have you been working in a program and performed some action that caused the program to abort and display a nasty err
wondering with which version of Python they should start. My answer to this question is usually something along the lines “just go with the version your favorite tutorial was written in, and check out the differences later on.” But what if you are starting a new project and have the choice to pick? I would say there is currently no “right” or “wrong” as long as both Python 2.7.x and Python 3.x support the libraries that you are planning to use. However, it is worthwhile to have a look at the major differences between those two most popular versions of Python to avoid common pitfalls when writing the code for either one of them, or if you are planning to port your project. Sections Sections The __future__ module The print function Python 2 Python 3 Integer division Python 2 Python 3 Unicode Python 2 Python 3 xrange Python 2 Python 3 The __contains__ method for range objects in Python 3 Note about the speed differences in Python 2 and 3 Raising exceptions Python 2 Python 3 Handling exceptions Python 2 Python 3 The next() function and .next() method Python 2 Python 3 For-loop variables and the global namespace leak Python 2 Python 3 Comparing unorderable types Python 2 Python 3 Parsing user inputs via input() Python 2 Python 3 Returning iterable objects instead of lists Python 2 Python 3 Banker’s Rounding Python 2 Python 3 More articles about Python 2 and Python 3 The __future__ module Python 3.x introduced some Python 2-incompatible keywords and features that can be imported via the in-built __future__ module in Python 2. It is recommended to use __future__ imports it if you are planning Python 3.x support for your code. For example, if we want Python 3.x’s integer division behavior in Python 2, we can import it via from __future__ import division More features that can be imported from the __future__ module are listed in the table below: feature optional in mandatory in effect nested_scopes 2.1.0b1 2.2 PEP 227: Statically Nested Scopes generators 2.2.0a1 2.3 PEP 255: Simple Generators division 2.2.0a2 3.0 PEP 238: Changing the Divisio