Python check if list items are integers

Question

I have a list which contains numbers and letters in string format   mylist   1   orange   2   3   4   apple     I need to come up with a new list which only contains numbers   mynewlist   1   2   3   4     If I have a way to check if each item in list can be converted to Integer  I should be able to come up with what I want by doing something like this   for item in mylist      if  check item can be converted to integer           mynewlist append item    How do I check that a string can be converted to an integer  Or is there any better way to do it

User · Answer

You can use exceptional handling as str digit will only work for integers and can fail for something like this too    gt  gt  gt  str isdigit   1   False   Using a generator function   def solve lis                                               for x in lis          try              yield float x          except ValueError                  pass   gt  gt  gt  mylist     1   orange   2   3   4   apple    1 5    2 6    gt  gt  gt  list solve mylist                                        1 0  2 0  3 0  4 0  1 5  2 6     returns converted values   or may be you wanted this   def solve lis       for x in lis          try              float x              return True         except              return False               gt  gt  gt  mylist     1   orange   2   3   4   apple    1 5    2 6    gt  gt  gt   x for x in mylist if solve x     1    2    3    4    1 5    2 6     or using ast literal eval  this will work for all types of numbers    gt  gt  gt  from ast import literal eval  gt  gt  gt  def solve lis       for x in lis          try              literal eval x              return True         except ValueError                  return False               gt  gt  gt  mylist   1   orange   2   3   4   apple    1 5    2 6    1 0j    gt  gt  gt   x for x in mylist if solve x                                    1    2    3    4    1 5    2 6    1 0j

User · Answer

Fast  simple  but maybe not always right    gt  gt  gt   x for x in mylist if x isdigit      1    2    3    4     More traditional if you need to get numbers   new list      for value in mylist      try          new list append int value       except ValueError          continue   Note  The result has integers  Convert them back to strings if needed  replacing the lines above with   try      new list append str int value

User · Answer

The usual way to check whether something can be converted to an int is to try it and see  following the EAFP principle   try      int value   int string value  except ValueError        it wasn t an int  do something appropriate else        it was an int  do something appropriate   So  in your case   for item in mylist      try          int value   int item      except ValueError          pass     else          mynewlist append item    or append int value  if you want numbers     In most cases  a loop around some trivial code that ends with mynewlist append item  can be turned into a list comprehension  generator expression  or call to map or filter  But here  you can t  because there s no way to put a try except into an expression   But if you wrap it up in a function  you can   def raises func   args    kw       try          func  args    kw      except          return True     else          return False  mynewlist    item for item in mylist if not raises int  item         or  if you prefer   mynewlist   filter partial raises  int   item      It s cleaner to use it this way   def raises exception types  func   args    kw       try          func  args    kw      except exception types          return True     else          return False   This way  you can pass it the exception  or tuple of exceptions  you re expecting  and those will return True  but if any unexpected exceptions are raised  they ll propagate out  So   mynewlist    item for item in mylist if not raises ValueError  int  item         will do what you want  but   mynewlist    item for item in mylist if not raises ValueError  item  int         will raise a TypeError  as it should

User · Answer

Try this  mynewlist    s for s in mylist if s isdigit      From the docs   str isdigit   Return true if all characters in the string are digits and there is at least one character  false otherwise  For 8-bit strings  this method is locale-dependent    As noted in the comments  isdigit   returning True does not necessarily indicate that the string can be parsed as an int via the int   function  and it returning False does not necessarily indicate that it cannot be  Nevertheless  the approach above should work in your case

[python] Python check if list items are integers?

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