在 Python 中什麼時候應該使用「try」以及什麼時候應該使用「if」？

Deciding between 'try' and 'if' Constructs in Python

When testing if a variable has a value, choosing between 'try' or 'if' constructs can be a matter of debate. Let's delve into the rationale behind using each approach.

EAFP (Easier to Ask for Forgiveness than Permission) vs. LBYL (Look Before You Leap)

Python encourages EAFP over LBYL. EAFP involves trying an operation and catching exceptions if they occur. LBYL, on the other hand, involves checking conditions upfront before attempting the operation.

Efficiency and Readability Considerations

The efficiency aspect depends on the expected frequency of exceptions. If exceptions are rare, the overhead of try/except blocks may be negated by the speed of if statements. However, if exceptions are more common, try/except may be faster as it avoids the overhead of condition checks in if statements.

Example

Consider the following code that checks if a function returns a list:

result = function()
if result:
    for r in result:
        # Process items

result = function()
try:
    for r in result:
        # Process items
except TypeError:
    pass

If 'result' is likely to be a list most of the time, the try/except approach is more efficient. However, if 'result' is often None, the if statement is preferable.

Time Measurements

Time measurements show that try/except is faster when exceptions are truly exceptional, while if statements are faster when conditions are commonly met.

Conclusion

The decision between 'try' and 'if' depends on:

• Expected frequency of exceptions
• Efficiency considerations
• Readability and coding style preferences

In general, EAFP (try/except) can be a more "pythonic" approach, especially when exceptions are rare. However, LBYL (if statements) may be more suitable when exceptions are common.

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