try clause vs if else clause

Question

Often, I come across cases where I find both if-else and try-except clauses can be used to solve a problem. As a result, I have some confusion deciding(and justifying) the use of a particular clause to accomplish the task at hand.
For example, let us consider a trivial situation:

In []: user = {"name": "Kshitij", "age": 20 }

# My motive is to print the user's email if it is available.
# In all other cases, a warning message needs to be printed

# Method-01 : Using try clause
In []: try:
  ...:     print(a["email"])
  ...: except KeyError:
  ...:     print("No EMAIL given")
  ...:     
No EMAIL given

# Method-02 : Using if-else clause
In []: if "email" in a:
           print(a["email"])
  ...: else:
  ...:     print("No EMAIL given")
  ...:     
No EMAIL given

I would like to know how can I decide a more Pythonic method among the two and justify it. Also, some pointers to how can one differentiate among several methods to solve similar scenarios would be really helpful.

This doesn't answer your question, but third alternative: `print(a.get("email", "No EMAIL given"))` ;-) — Kevin, Jan 11 '17 at 19:42
I don't think this is a duplicate. In my opinion, this is an amalgamation of all such cases. I just happened to give an example which is related to another question. — Kshitij Saraogi, Jan 11 '17 at 19:48
@Kevin Won't you agree with me if I say that "getting" `No EMAIL given` is a bad Pythonic construct since the motive here is not to "get" it, rather display it ? — Kshitij Saraogi, Jan 11 '17 at 19:50

score 2 · Answer 1 · answered Jan 11 '17 at 19:41

try/catch and if/else are not interchangeable. the former is for catching errors that might be thrown. no if statement can do that. if/else is for checking if a condition is true or false. errors thrown in an if/else block will not be caught and the program will crash.

score 0 · Answer 2 · answered Jan 11 '17 at 19:41

0

You should use exceptions if this case is an exception and not the normal case.

answered Jan 11 '17 at 19:41

Daniel

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score 0 · Answer 3 · answered Jan 11 '17 at 19:43

0

if/then: for internal application flow control. Errors are unlikely to be fatal

vs

try/except: for system calls and API calls where the state of the receiver is external to the code. Failures are likely to be fatal and need to handled explicitly

answered Jan 11 '17 at 19:43

Manager_of_it

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score 0 · Answer 4 · answered Jan 11 '17 at 19:47

If you're looking for "Pythonic", neither of those is.

print a["email"] if "email" in a else "No EMAIL given"

Now that's Pythonic. But, back to the question: First of all, you decide what to do - I'm not aware of any writing conventions between those two. But, as I see it:

if-else is used mainly for detecting expectable behaviour - I mean, if "email" is not in a, it's excpectable. So we will use if-else.
An example would be your code.

But, for example, if we want to check if a string contains a numeric value, we will try to convert it to a number. If it failed, well, it's a bit harder to predict it using an if statement, so we will use try-except.
Here's a short example for that:

def is_numeric(string):
    try:
        a = float(string)
        return True
    except:
        return False

of course there is a better way to check if a string is numeric, that was just an example use of try-except.

try clause vs if else clause

4 Answers4