Check element exists in array

In PHP there a function called isset() to check if something (like an array index) exists and has a value. How about Python?

I need to use this on arrays because I get "IndexError: list index out of range" sometimes.

I guess I could use try/catching, but that's a last resort.

Look before you leap (LBYL):

if idx < len(array):
    array[idx]
else:
    # handle this

Easier to ask forgiveness than permission (EAFP):

try:
    array[idx]
except IndexError:
    # handle this

In Python, EAFP seems to be the popular and preferred style. It is generally more reliable, and avoids an entire class of bugs (time of check vs. time of use). All other things being equal, the try/except version is recommended - don't see it as a "last resort".

This excerpt is from the official docs linked above, endorsing using try/except for flow control:

This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. This clean and fast style is characterized by the presence of many try and except statements.

EAFP vs. LBYL

I understand your dilemma, but Python is not PHP and coding style known as Easier to Ask for Forgiveness than for Permission (or EAFP in short) is a common coding style in Python.

See the source (from documentation):

EAFP - Easier to ask for forgiveness than permission. This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. This clean and fast style is characterized by the presence of many try and except statements. The technique contrasts with the LBYL style common to many other languages such as C.

So, basically, using try-catch statements here is not a last resort; it is a common practice.

"Arrays" in Python

PHP has associative and non-associative arrays, Python has lists, tuples and dictionaries. Lists are similar to non-associative PHP arrays, dictionaries are similar to associative PHP arrays.

If you want to check whether "key" exists in "array", you must first tell what type in Python it is, because they throw different errors when the "key" is not present:

>>> l = [1,2,3]
>>> l[4]

Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    l[4]
IndexError: list index out of range
>>> d = {0: '1', 1: '2', 2: '3'}
>>> d[4]

Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    d[4]
KeyError: 4

And if you use EAFP coding style, you should just catch these errors appropriately.

LBYL coding style - checking indexes' existence

If you insist on using LBYL approach, these are solutions for you:

• for lists just check the length and if possible_index < len(your_list), then your_list[possible_index] exists, otherwise it doesn't:

  >>> your_list = [0, 1, 2, 3]
  >>> 1 < len(your_list) # index exist
  True
  >>> 4 < len(your_list) # index does not exist
  False
  

• for dictionaries you can use in keyword and if possible_index in your_dict, then your_dict[possible_index] exists, otherwise it doesn't:

  >>> your_dict = {0: 0, 1: 1, 2: 2, 3: 3}
  >>> 1 in your_dict # index exists
  True
  >>> 4 in your_dict # index does not exist
  False
  

Did it help?