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Question

This is a homework problem. I've been trying to solve it but couldn't get the right result.

This is the question:

Write a function string2int that attempts to convert a string to an integer. If the string does represent a positive integer then that integer should be returned. If the string does not represent a positive integer then you should raise a syntax exception ( raise SyntaxError('not an integer') ).

You may choose to use the (already defined) function all_digits that takes a string and returns True if all the characters of the string are digits and False otherwise.

What I've got so far is:

try all_digits is True:
    return int(num)
except all_digits is False:
    raise SyntaxError('not an integer')

Because I'm using an already defined function, I didn't define a function (or did I get it wrong?). Can anyone have a look at my code please? Much appreciated.

Answer 1

I can guess, but you might want to tell us what kind of error you get when you execute the code (just a heads up for the next time you ask a question).

There's a couple of mistakes:

1) The syntax of

try all_digits is True:

is wrong. The "try" statement should look like this:

try:
    <your code>
except <type of exception to catch>:
    <error handling code>

2) You said "all_digits" is a function. Therefore, the code

all_digits is True

should be

if all_digits(num):

Putting it all together:

def string2int(num):
    if all_digits(num):
        return int(num)
    raise SyntaxError('not an integer')

Answer 2

In addition to Rawing's excellent answer, note that the usual time to use try/except is when you can handle the error thrown in the try block and continue as usual. For instance:

def add_three(x) -> int:
    try:
        return x + 3
    except TypeError:
        # someone passed a non-int/float to the function!
        #   let's try and coerce it.
        return int(x) + 3
        # if this still throws an exception, we'll let it
        #   raise its own TypeError exception!

In your case it looks like you're just doing regular conditionals, so use if all_digits(num): return int(num) else: raise TypeError('not an integer')

Answer 3

all_digits(string) function:

First, it's good to understand what does the pre-defined all_digits(string) function do. Following is a sample implementation of that function, which works as desired by your description. It checks whether each letter of the string is a digit and returns a boolean, True or False , accordingly:

def all_digits(string):
''' only returns True if all characters of the string are Integers '''
    for l in string:
        if l.isdigit(): pass
        else: return False
    return True

string2num(string) function with raise statement:

Now, we can use this function in our error handling block of the string2num(string) function. Since your problem requires you to only raise a specific exception and not to continue with an alternate block of code, you do not need the try: ... except: block.

With the proper syntax of the raise statement , we can write:

def string2num( string = '-23'):
    if all_digits(string):
        return int('23')
    raise SyntaxError("not an integer")

and we get:

>>> string2num()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 4, in string2num
SyntaxError: not an integer

with try: ... except: ... block:

But if you do want to execute an alternate block of code when the exception is raised, you can use the try: ... except: block syntax. You may need it, for instance, if you want to further check if the string is a negative integer and if so then return the negative integer:

def string2num( string = '-23'):
    try:
        if all_digits(string):
            return int(string)
        raise SyntaxError("not an integer")
    except SyntaxError:
        #alternate code goes here#
        try:
            return int(string)
        except ValueError:
            print "string contains an alphabet"

This will produce:

>>> string2num()
-23
>>> string2num('ab2')
string contains an alphabet

Style for if statement:

As a side note on your style, you don't have to explicitly write whether an expression evaluates to True or False in an if statement, like so:

if all_digits(string) is True:

Since all_digits(string) returns a boolean, you can equivalently just say if True , like so:

if all_digits(string):