python - How do I parse a string to a float or int?

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Top 5 Answer for python - How do I parse a string to a float or int?

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95

>>> a = "545.2222" >>> float(a) 545.22220000000004 >>> int(float(a)) 545 
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86

Python method to check if a string is a float:

def is_float(value):   try:     float(value)     return True   except:     return False 

A longer and more accurate name for this function could be: is_convertible_to_float(value)

What is, and is not a float in Python may surprise you:

val                   is_float(val) Note --------------------  ----------   -------------------------------- ""                    False        Blank string "127"                 True         Passed string True                  True         Pure sweet Truth "True"                False        Vile contemptible lie False                 True         So false it becomes true "123.456"             True         Decimal "      -127    "      True         Spaces trimmed "\t\n12\r\n"          True         whitespace ignored "NaN"                 True         Not a number "NaNanananaBATMAN"    False        I am Batman "-iNF"                True         Negative infinity "123.E4"              True         Exponential notation ".1"                  True         mantissa only "1,234"               False        Commas gtfo u'\x30'               True         Unicode is fine. "NULL"                False        Null is not special 0x3fade               True         Hexadecimal "6e7777777777777"     True         Shrunk to infinity "1.797693e+308"       True         This is max value "infinity"            True         Same as inf "infinityandBEYOND"   False        Extra characters wreck it "12.34.56"            False        Only one dot allowed u'四'                 False        Japanese '4' is not a float. "#56"                 False        Pound sign "56%"                 False        Percent of what? "0E0"                 True         Exponential, move dot 0 places 0**0                  True         0___0  Exponentiation "-5e-5"               True         Raise to a negative number "+1e1"                True         Plus is OK with exponent "+1e1^5"              False        Fancy exponent not interpreted "+1e1.3"              False        No decimals in exponent "-+1"                 False        Make up your mind "(1)"                 False        Parenthesis is bad 

You think you know what numbers are? You are not so good as you think! Not big surprise.

Don't use this code on life-critical software!

Catching broad exceptions this way, killing canaries and gobbling the exception creates a tiny chance that a valid float as string will return false. The float(...) line of code can failed for any of a thousand reasons that have nothing to do with the contents of the string. But if you're writing life-critical software in a duck-typing prototype language like Python, then you've got much larger problems.

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75

def num(s):     try:         return int(s)     except ValueError:         return float(s) 
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65

This is another method which deserves to be mentioned here, ast.literal_eval:

This can be used for safely evaluating strings containing Python expressions from untrusted sources without the need to parse the values oneself.

That is, a safe 'eval'

>>> import ast >>> ast.literal_eval("545.2222") 545.2222 >>> ast.literal_eval("31") 31 
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57

float(x) if '.' in x else int(x) 

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