Thứ hai Tháng một 10 01. 22. 33 CET 2011
Karim wrote: > > Hello all, > > I am using more and more try except statement. But I discussed with a > very experienced C++ programmer. > And he told me that at least in C++ using too many try -catch statements > are making the code slower. And it should > does the same for python [C implementation]. He is right with the first part, wrong with the second. Python's exceptions are not the same as C++ or Java exceptions. Setting up a try...except block is *very* fast, unlike C++ and Java, where it is slow. In Python, this code: try: 1+1 # cannot fail except ValueError: pass is just as fast as: pass # null op, does nothing 1+1 # cannot fail So there is no speed penalty for setting up a try...except block. If you have something that will nearly always succeed, you don't need to be concerned about using an exception handler. However, *catching* the exception is more expensive. [Roughly ten times as expensive.] So you should avoid code that mostly fails. Remember, also, that the alternative to try...except also has a cost which might not be cheap. You can do this: try: something that might fail except Failure: do alternative or you can do this: if something should succeed: do something else: do alternative But this if...else test is not free. Depending on how expensive the test is, and how often you take each branch, it might be more expensive to "look before you leap". Also, remember that many tests are disguised exceptions, such as the builtin "hasattr" function: # pseudo-code def hasattr[obj, name]: try: getattr[obj, name] except AttributeError: return False return True So there is no speed advantage to calling: if hasattr[obj, 'spam']: print[obj.spam] else: print["no spam in object"] over this: try: print[obj.spam] except AttributeError: print["no spam in object"] The first may even be slower because it has to look up the attribute twice -- this is what I call a pessimation, something done in the name of speed which actually makes your code *slower*. Catching exceptions are best in three circumstances: - You only care about the speed of a successful operation. Failures can be slow. If your program prints an error message and then exits, who cares if it takes 3 milliseconds longer than necessary? - Testing whether something will succeed is impossible, or expensive, or too hard to get right. - Failures are rare. As a *very* rough rule of thumb, I say that if you expect a failure less than one time in ten, it is better to use try...except. But mostly, don't worry about speed. If you are worried about *execution* speed, you shouldn't be using Python in the first place. Python is optimized for developer productivity, not execution speed. Write your code, and *if* it is too slow, then worry about finding the bottlenecks and optimizing them. Some very important quotes about optimization in general: “More computing sins are committed in the name of efficiency [without necessarily achieving it] than for any other single reason - including blind stupidity.” - W.A. Wulf “We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we should not pass up our opportunities in that critical 3%. A good programmer will not be lulled into complacency by such reasoning, he will be wise to look carefully at the critical code; but only after that code has been identified.” - Donald Knuth “Bottlenecks occur in surprising places, so don't try to second guess and put in a speed hack until you have proven that's where the bottleneck is.” - Rob Pike “The First Rule of Program Optimization: Don't do it. The Second Rule of Program Optimization [for experts only!]: Don't do it yet.” - Michael A. Jackson -- StevenThông tin thêm về danh sách gửi thư Tutor
Khi xảy ra lỗi hoặc ngoại lệ như chúng ta gọi, Python thường sẽ dừng và tạo thông báo lỗi
Những ngoại lệ này có thể được xử lý bằng cách sử dụng câu lệnh try
Thí dụ
Khối try
sẽ tạo ra một ngoại lệ, vì x
không được xác định
thử.
print[x]
ngoại trừ.
print["Đã xảy ra ngoại lệ"]
Vì khối try phát sinh lỗi nên khối except sẽ được thực thi
Nếu không có khối thử, chương trình sẽ bị lỗi và gây ra lỗi
Thí dụ
Câu lệnh này sẽ gây ra lỗi, vì x
không được xác định
Nhiều ngoại lệ
Bạn có thể xác định bao nhiêu khối ngoại lệ tùy thích, e. g. nếu bạn muốn thực thi một khối mã đặc biệt cho một loại lỗi đặc biệt
Thí dụ
In một thông báo nếu khối thử tăng 5 và một thông báo khác cho các lỗi khác
thử.
print[x]
ngoại trừ NameError.
print["Biến x không được xác định"]
ngoại trừ.
print["Đã xảy ra sự cố khác"]
Khác
Bạn có thể sử dụng từ khóa else
để xác định một khối mã sẽ được thực thi nếu không có lỗi nào được đưa ra
Thí dụ
Trong ví dụ này, khối try
không tạo ra bất kỳ lỗi nào
thử.
print["Xin chào"]
ngoại trừ.
print["Đã xảy ra sự cố"]
else.
print["Không có gì sai"]
Cuối cùng
Khối finally
, nếu được chỉ định, sẽ được thực thi bất kể khối thử có gây ra lỗi hay không