Check element exists in array

Question

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

User · Answer

has key is fast and efficient   Instead of array use an hash   valueTo1   a   b   c    if valueTo1 has key  a            print  Found key in dictionary

User · Answer

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    gt  gt  gt  l    1 2 3   gt  gt  gt  l 4   Traceback  most recent call last     File   lt pyshell 2 gt    line 1  in  lt module gt      l 4  IndexError  list index out of range  gt  gt  gt  d    0   1   1   2   2   3    gt  gt  gt  d 4   Traceback  most recent call last     File   lt pyshell 6 gt    line 1  in  lt module gt      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  lt  len your list   then your list possible index  exists  otherwise it doesn t    gt  gt  gt  your list    0  1  2  3   gt  gt  gt  1  lt  len your list    index exist True  gt  gt  gt  4  lt  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    gt  gt  gt  your dict    0  0  1  1  2  2  3  3   gt  gt  gt  1 in your dict   index exists True  gt  gt  gt  4 in your dict   index does not exist False    Did it help

User · Answer

In Python you might be in for some surprises if you ask for forgiveness in this case   try-except is not the right paradigm here    If you accidentally get negative indices your in for a surprise    Better solution is to provide the test function yourself   def index in array M  index       return index 0   gt   0 and index 1   gt   0 and index 0  lt  M shape 0  and index 1   lt  M shape 1

User · Answer

Look before you leap  LBYL   if idx  lt  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  quot last resort quot   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

User · Answer

You may be able to use the built-in function dir   to produce similar behavior to PHP s isset    something like   if  foo  in dir       returns False  foo is not defined yet      pass  foo    b   if  foo  in dir       returns True  foo is now defined and in scope     pass   dir   returns a list of the names in the current scope  more information can be found here  http   docs python org library functions html dir

User · Answer

e  in   a    b    c      evaluates as False  b  in   a    b    c      evaluates as True   EDIT  With the clarification  new answer   Note that PHP arrays are vastly different from Python s  combining arrays and dicts into one confused structure  Python arrays always have indices from 0 to len arr  - 1  so you can check whether your index is in that range  try catch is a good way to do it pythonically  though   If you re asking about the hash functionality of PHP  arrays   Python s dict   then my previous answer still kind of stands    baz  in   foo   17   bar   19     evaluates as False  foo  in   foo   17   bar   19     evaluates as True

[python] Check element exists in array

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