Converting String to Int Using Try/Except in Python

How can I check if a string represents an int, without using try/except?

If you're really just annoyed at using try/excepts all over the place, please just write a helper function:

def RepresentsInt(s):
    try: 
        int(s)
        return True
    except ValueError:
        return False

>>> print RepresentsInt("+123")
True
>>> print RepresentsInt("10.0")
False

It's going to be WAY more code to exactly cover all the strings that Python considers integers. I say just be pythonic on this one.

Why am I unable to use try-except to convert str to int?

I often use a function to safely convert a string to an int or a float, e.g. to_int below.

The code below creates an answer Label that's used as required. The original code created a label for every click of the button.

from tkinter import *

def to_int(string, fail = 0):
    try: 
        return int(string)
    except ValueError:
        return fail

x = {"A": 0.8, "B": 2.5, "C": 1.2}

def p(y):
    root = Tk()
    root.title("Average Waiting Time")
    root.geometry("800x500")

    Label(root, text="Enter number of people: ").place(x=20, y=30)
    Q = Entry(root, bd =5)
    Q.place(x=220, y=30)

    answer = StringVar()
    answer.set("")
    Label(root, textvariable = answer ).place(x=50, y=500)

    def B():
        Q_input = Q.get()

        v = to_int(Q_input, -1)
        # Returning <0 triggers the error message

        if v <=0 or v > 10:
            answer.set("Enter a value from 0 to 10")
        else:               
            W = str(round(v * x[y], 1))  # Round off to 1 d.p
            answer.set("Ans\n" + W + " minute(s)")
            print(answer.get())

    ok = Button (root, text="OK", command = B)
    ok.place(x=300, y=300)

    cancel = Button (root, text="Cancel", command = root.destroy)
    cancel.place(x=400,y=300)

    root.mainloop()

Exception Handling, function attempting to convert a string into an integer

def string2int(sti):
    try:
        return int(sti)
    except ValueError:
        raise SyntaxError('not an integer')

Or:

def string2int(sti):
    if sti.isdigit():
        return int(sti)
    else:
        raise SyntaxError('not an integer')

Trying to convert to numbers with try except

try-except is for catching exceptions. In this scenario, you only account for one exception, ValueError but not TypeError. In order to catch type error, just put one more except block below try. In your case, it would be like this:

L = ["29.3", "tea", "1", None, 3.14]
D = []
for item in L:
    try:
        float(item)
        D.append(float(item))
    except TypeError:
        # do something with None here
    except ValueError:
        D.append(item)

print(D)

Given that you want to catch multiple exceptions in a single except block, use a tuple of exceptions:

L = ["29.3", "tea", "1", None, 3.14]
D = []
for item in L:
    try:
        float(item)
        D.append(float(item))
    except (ValueError, TypeError):
        D.append(item)
print(D)

Is there a built-in or more Pythonic way to try to parse a string to an integer

This is a pretty regular scenario so I've written an "ignore_exception" decorator that works for all kinds of functions which throw exceptions instead of failing gracefully:

def ignore_exception(IgnoreException=Exception,DefaultVal=None):
    """ Decorator for ignoring exception from a function
    e.g.   @ignore_exception(DivideByZero)
    e.g.2. ignore_exception(DivideByZero)(Divide)(2/0)
    """
    def dec(function):
        def _dec(*args, **kwargs):
            try:
                return function(*args, **kwargs)
            except IgnoreException:
                return DefaultVal
        return _dec
    return dec

Usage in your case:

sint = ignore_exception(ValueError)(int)
print sint("Hello World") # prints none
print sint("1340") # prints 1340

Try Except String vs Int Python

this solution counts on the fact that int("1.235") will raise a value error as for a string to convert it must be a literal int. This requires my_value to be a string! as int(1.235) will simply truncate the float to an int

my_value = raw_input("Enter Value")

try:
  my_value = int(my_value)
except ValueError:
  try:
     float(my_value)
     print "Its a float not an int!"
     raise ValueError("Expected Int, got Float!")
  except ValueError:
     print "Its a string not a float or int"
     raise TypeError("Expected Int, got String!")
else:
  print "OK its an int"

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