Tutorial :converting python list of strings to their type



Question:

given a list of python strings, how can I automatically convert them to their correct type? Meaning, if I have:

["hello", "3", "3.64", "-1"]  

I'd like this to be converted to the list

["hello", 3, 3.64, -1]    

where the first element is a stirng, the second an int, the third a float and the fourth an int.

how can I do this? thanks.


Solution:1

import ast    L = ["hello", "3", "3.64", "-1"]    def tryeval(val):    try:      val = ast.literal_eval(val)    except ValueError:      pass    return val    print [tryeval(x) for x in L]  


Solution:2

Without using evaluation:

def convert(val):      constructors = [int, float, str]      for c in constructors:          try:              return c(val)          except ValueError:              pass  


Solution:3

def tryEval(s):    try:      return eval(s, {}, {})    except:      return s    map(tryEval, ["hello", "3", "3.64", "-1"])  

Only do this if you trust the input. Also, be aware that it supports more than just literals; arithmetic expressions will be evaluated as well.


Solution:4

I accomplished the same using json.loads method

def f(l):      for i in l:          try:              yield json.loads(i)          except:              yield i  

Test:

In [40]: l  Out[40]: ['hello', '3', '3.64', '-1']    In [41]: list(f(l))  Out[41]: ['hello', 3, 3.64, -1]  


Solution:5

If the you are truly interested in only strings, floats, and ints, I prefer the more verbose, less-evalful

def interpret_constant(c):      try:          if str(int(c)) == c: return int(c)      except ValueError:          pass      try:          if str(float(c)) == c: return float(c)      except ValueError:          return c    test_list = ["hello", "3", "3.64", "-1"]    typed_list = [interpret_constant(x) for x in test_list]  print typed_list  print [type(x) for x in typed_list]  


Solution:6

This is not really an answer, but I wanted to point out how important this can be when you have a database of parameters with schema ID, PAR, VAL. For instance:

ID  PAR      VAL  001 velocity '123.45'  001 name     'my_name'  001 date     '18-dec-1978'  

This schema is appropriate when you don't know how many parameters you need to store for a certain ID. The disadvantage is precisely that the values in VAL are all strings, and need to be converted to the correct data type on demand. You can do this by adding a fourth column to the schema, called TYPE, or you can use any of the approaches proposed thus far.

Good question!

PS. The database schema is related to one of my previous questions.


Solution:7

A variant of ryans's nice solution, for numpy users:

def tonum( x ):      """ -> int(x) / float(x) / None / x as is """      if np.isscalar(x):  # np.int8 np.float32 ...      # if isinstance( x, (int, long, float) ):          return x      try:          return int( x, 0 )  # 0: "0xhex" too      except ValueError:          try:              return float( x )  # strings nan, inf and -inf too          except ValueError:              if x == "None":                  return None              return x    def numsplit( line, sep=None ):      """ line -> [nums or strings ...] """      return map( tonum, line.split( sep ))  # sep None: whitespace  

Note:If u also have question or solution just comment us below or mail us on toontricks1994@gmail.com
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