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Asked: 2026-06-15T04:01:34+00:00

I would like to try to deduce the type of data in a string.

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I would like to try to deduce the type of data in a string.

Scenario:
I have a CSV file which contains rows of data, and I would like to store this data in a database.
I do not want to store all the fields as strings.
Since the fields in the CSV might change, I cannot assume anything about their types.

Example (CSV file):

[Row 1 - column names] --> "name", "age" , "children"
[Row 2 - data row    ] --> "John", "45.5", "3"
...
[Row n - data row    ] --> ...

In this case, by looking at the data in the rows, I would like to deduce that name is a column of strings, age is a column of floats and children is a column of integers.

My attempt:
The simplest approach would be to try conversions, and decide upon the type when a certain conversion succeeds.
I wrote a method for this purpose which looks like this:

def deduceType(str):
    try:
        #first try to convert to int:
        int(str)
        return 0 #integer
    except ValueError:
        try:
            #not integer, try float:
            float(str)
            return 1 #float
        except ValueError:
            #not float, so deduct string
            return 2 #string

My question:
The problem is that if I want to be able to deduce more data types (booleans, longs, unsigned numeric types, etc…), then this approach becomes cumbersome and inaccurate.

Is there a neater, more efficient and rigorous way to do this?

Answer (edit):
Based on Martijn Pieters answer, I’m doing this:

def deduceType(str):
    try:
        return type(ast.literal_eval(str))
    except ValueError:
        return type('') #string
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1 Answer

  1. Editorial Team

    Use ast.literal_eval() on the value; it’ll interpret it as a python literal. If that fails, you have a string instead.

    >>> import ast
>>> ast.literal_eval("45.5")
45.5
>>> ast.literal_eval("3")
3
>>> ast.literal_eval("John")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/ast.py", line 68, in literal_eval
    return _convert(node_or_string)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/ast.py", line 67, in _convert
    raise ValueError('malformed string')
ValueError: malformed string
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