Operations¶

try / except / finally
raise
assert
with / as

Usage¶

Error Handling
- Catch and response to errors
- Ignore errors (or Python will trigger default handling behavior)
Event Notification
- Singal valid condition without pass result flags
Special-case Handling
Termination Actions
- Guarantee closing-time operation be performed (with/as can be an alternative)
Unusual Control Flow
- High-Level and structured "go to" (raise can be used as jump)

In [2]:

# Default Exception Handler
a

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-2-06bae13eb8cf> in <module>()
      1 # Default Exception Handler
----> 2 a

NameError: name 'a' is not defined

In [3]:

try:
    a
except NameError:
    print("a is not defined")
finally:
    print("Runs no matter exception occur or not")

a is not defined
Runs no matter exception occur or not

There is no way in Python to go back to the code that triggered the exception
assert is a conditional raise, used mostly for debugging

In [4]:

assert 1 == 1

In [5]:

assert 1 == 2

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-5-a810b3a4aded> in <module>()
----> 1 assert 1 == 2

AssertionError:

Context Management (with/as)¶

An alternative to try / except , but can only be used for some types of oject
Guarantee termination actions
It supports user-defined contect mangement

In [6]:

with open("../README.md") as f:
    pass

The above code perform both open() and close()