Python Try Except
The try block lets you test a
block of code for errors.
The except block lets you
handle the error.
The else block lets you
execute code when there is no error.
The finally block lets you
execute code, regardless of the result of the try- and except blocks.
Exception Handling
When an error occurs, or exception as we call it, Python will normally stop and generate an error message.
These exceptions can be handled using the try statement:
Example
The try block will generate an exception,
because x is not defined:
try:
print(x)
except:
print("An exception occurred")
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Since the try block raises an error, the except block will be executed.
Without the try block, the program will crash and raise an error:
Many Exceptions
You can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error:
Example
Print one message if the try block raises a NameError and another
for other errors:
try:
print(x)
except NameError:
print("Variable x
is not defined")
except:
print("Something else went
wrong")
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Else
You can use the else keyword to define a
block of code to be executed if no errors were raised:
Example
In this example, the try block does not
generate any error:
try:
print("Hello")
except:
print("Something went
wrong")
else:
print("Nothing went wrong")
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Finally
The finally block, if specified, will be executed
regardless if the try block
raises an error or not.
Example
try:
print(x)
except:
print("Something went
wrong")
finally:
print("The 'try except' is finished")
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This can be useful to close objects and clean up resources:
Example
Try to open and write to a file that is not writable:
try:
f = open("demofile.txt")
try:
f.write("Lorum Ipsum")
except:
print("Something went wrong when writing to the file")
finally:
f.close()
except:
print("Something went wrong when opening the
file")
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The program can continue, without leaving the file object open.
Raise an exception
As a Python developer you can choose to throw an exception if a condition occurs.
To throw (or raise) an exception, use the raise keyword.
Example
Raise an error and stop the program if x is lower than 0:
x = -1
if x < 0:
raise Exception("Sorry, no numbers below
zero")
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The raise keyword is used to raise an
exception.
You can define what kind of error to raise, and the text to print to the user.
Example
Raise a TypeError if x is not an integer:
x = "hello"
if not type(x) is int:
raise TypeError("Only
integers are allowed")
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