Errors and Exceptions¶
Until now error messages haven’t been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors:
Syntax errors
Exceptions
Syntax Errors (Parsing Errors)¶
while True print('Ohh Man!, not again')
File "<ipython-input-1-a34048f0cb3e>", line 1
while True print('Ohh Man!, not again')
^
SyntaxError: invalid syntax
The parser repeats the offending line and displays a little ‘arrow’ pointing at the earliest point in the line where the error was detected. The error is caused by (or at least detected at) the token preceding the arrow:
In the example, the error is detected at the function print(), since a colon (‘:’) is missing before it. File name and line number are printed so you know where to look in case the input came from a script.
Exceptions¶
Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions
10 * (1/0)
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-3-0b280f36835c> in <module>
----> 1 10 * (1/0)
ZeroDivisionError: division by zero
4 + spam*3
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
<ipython-input-4-c98bb92cdcac> in <module>
----> 1 4 + spam*3
NameError: name 'spam' is not defined
'2' + 2
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-5-d2b23a1db757> in <module>
----> 1 '2' + 2
TypeError: can only concatenate str (not "int") to str
Handling Exception¶
Exceptions ca be handled using the try..except statement. You basically put all your statements in try block and put error handlers in except block
while True:
try:
x = int(input("Please enter a number: "))
break
except ValueError:
print("Oops! That was no valid number. Try again...")
Please enter a number: $2
Oops! That was no valid number. Try again...
Please enter a number: 2
raise Exceptions¶
The raise statement allows the programmer to force a specified exception to occur. For example:
raise NameError('How you doin?')
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
<ipython-input-7-6a59564f9d69> in <module>
----> 1 raise NameError('How you doin?')
NameError: How you doin?
If you need to determine whether an exception was raised but don’t intend to handle it, a simpler form of the raise statement allows you to re-raise the exception:
try:
raise NameError('How you doin?')
except NameError:
print('An exception flew by!, How you doin?')
raise
An exception flew by!, How you doin?
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
<ipython-input-8-4bd984bb795d> in <module>
1 try:
----> 2 raise NameError('How you doin?')
3 except NameError:
4 print('An exception flew by!, How you doin?')
5 raise
NameError: How you doin?
Let’s clean up¶
The try statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:
try:
raise NameError('How you doin?')
finally:
print('Goodbye, world!')
Goodbye, world!
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
<ipython-input-9-5e7cc627406e> in <module>
1 try:
----> 2 raise NameError('How you doin?')
3 finally:
4 print('Goodbye, world!')
NameError: How you doin?
Built-In Exceptions¶
Create your own exception?¶
Revisit later