You will face at least two types of errors while coding with python. Syntax errors and exceptions.
Syntax errors are also known as parsing errors. Compiler informs us about the parsing errors using an arrow.
rana@brahma:~$ python3 Python 3.5.2 (default, Nov 12 2018, 13:43:14) [GCC 5.4.0 20160609] on linux Type "help", "copyright", "credits" or "license" for more information. >>> while True print("Hello") File "<stdin>", line 1 while True print("Hello") ^ SyntaxError: invalid syntax >>>
Even when your code is perfect syntax wise, some lines may report an error when you try to execute them. Errors reported at runtime are known as exceptions.
For an uninterrupted execution, we must handle the exceptions properly. We can do so by using
In below example, we are trying to add a string and an integer. When the statement is executed it throws an exception.
>>> a = 'rana' + 10 Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: Can't convert 'int' object to str implicitly >>>
If we don't handle this exception, the program will exit abruptly. Handle the exception using
>>> >>> try: ... a = 'rana' + 10 ... except Exception as e: ... print('Some useful message to debug the code') ... Some useful message to debug the code >>>
When the above piece of code is executed, an exception is thrown but is then caught by
except block which in turn print a useful message which helps in debugging the code.
try ... except statement have an optional
else clause. Else part is executed when there is no exception thrown in
try clause. Else part is executed before
Let's say you are trying to open a file in try block (just an example) and it is possible there might occur some error in opening the file, we will handle exception in except block. If there is no exception thrown, the file is opened successfully, we have to close the file descriptor.
>>> try: ... f = open('filename.txt','r') ... except Exception as e: ... print('Some exception in opening the file') ... else: ... f.close() ...
It's better to add code to
else clause instead of adding code to
try clause. This helps in avoiding catching the exception which was not raised by code being protected in
try caluse. For example in addition to opening file in
try clause, we were also trying to convert a variable to an integer. Imaging file opened just fine but conversion to int threw an exception which will be reported by except block as an exception in opening file, which will be misleading. See the example code below.
>>> try: ... f = open('filename.txt','r') ... a = 'five' ... b = int(a) ... except Exception as e: ... print('can not open file') ... else: ... f.close() ... can not open file >>>
Above could be rewritten as below.
try: f = open('filename.txt','r') print('file was opened') except Exception as e: print('can not open file') else: print('going to close file') f.close() try: a = 'five' b = int(a) except Exception as e: print('exception in number conversion')
file was opened going to close file exception in number conversion
elseblock to control the execution flow?
stopIterationexception to signal the end of items.
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