How to tell if string starts with a number with Python?

Question

I have a string that starts with a number (from 0-9) I know I can "or" 10 test cases using startswith() but there is probably a neater solution

so instead of writing

if (string.startswith('0') || string.startswith('2') ||
    string.startswith('3') || string.startswith('4') ||
    string.startswith('5') || string.startswith('6') ||
    string.startswith('7') || string.startswith('8') ||
    string.startswith('9')):
    #do something

Is there a cleverer/more efficient way?

If the question is asked: "Is this too repetitive?", the chances are -- in a high level language -- the answer is "Why, yes, it sure is". Be lazy! — user166390
– user166390, Commented Apr 7, 2011 at 7:46
@Illusionist As it is written, your question means that you want to detect the strings that begin with only ONE digit. If so, the only right answer among the following ones, are not the ones using s[0] or s[:1] but the solution of John Machin: if s.startswith(tuple('0123456789')). Moreover, this solution raises error when it happens that s is a sequence like a tuple or list, which cases produce the same result as if it was a string. - Another solution is a regex whose pattern is '\d(?=\D)' but use of regex is superfluous here. — eyquem
– eyquem, Commented Aug 20, 2011 at 11:10
Just being pedantic: string is a module in the standard library and probably shouldn't be used as a variable name. docs.python.org/2/library/string.html — gak
– gak, Commented Mar 14, 2013 at 23:16

plaes · Accepted Answer · 2011-04-07 07:37:03Z

259

Python's string library has isdigit() method:

string[0].isdigit()

answered Apr 7, 2011 at 7:37

plaes

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3 Comments

John Machin Over a year ago

Python2's string module doesn't have methods, and isdigit is a method of str and unicode objects.

John Machin Over a year ago

@plaes: -1 because of the above PLUS it crashes if s == ""

PascalVKooten Over a year ago

Look at my answer which does not break for an empty string: string[:1].isdigit()

Jaroslav Bezděk · Accepted Answer · 2019-11-16 14:08:56Z

38

>>> string = '1abc'
>>> string[0].isdigit()
True

edited Nov 16, 2019 at 14:08

Jaroslav Bezděk

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answered Apr 7, 2011 at 7:37

jcomeau_ictx

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6 Comments

corsiKa Over a year ago

I'm not a python guy, so perhaps you can help me on this: will this function bomb on "", where there is no string[0]?

jcomeau_ictx Over a year ago

yep, it sure will. you could use (string or 'x')[0].isdigit() to fix it for '' or None

Simon Callan Over a year ago

You could try string[:1].isdigit(), which will quit happily deal with an empty string.

jcomeau_ictx Over a year ago

because that's not my answer; mine is still better in the case where the programmer wishes an exception to be thrown on an empty string.

james-see Over a year ago

@jcomeau_ictx exactly right. never defensively program. if the string is unexpectedly blank, that should error out and shouldn't be automatically handled anyway. Some schools of thought say you should gracefully handle and catch exceptions that aren't critical as warning and keep the program going. I am not of this mindset for most use cases. Specificity in the functions and code you are writing helps mitigate the edge cases where you would need to code defensively more often.

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PascalVKooten · Accepted Answer · 2019-08-09 14:11:10Z

32

Surprising that after such a long time there is still the best answer missing.

The downside of the other answers is using [0] to select the first character, but as noted, this breaks on the empty string.

Using the following circumvents this problem, and, in my opinion, gives the prettiest and most readable syntax of the options we have. It also does not import/bother with regex either):

>>> string = '1abc'
>>> string[:1].isdigit()
True

>>> string = ''
>>> string[:1].isdigit()
False

answered Aug 9, 2019 at 14:11

PascalVKooten

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Comments

kurumi · Accepted Answer · 2011-04-07 07:38:55Z

12

sometimes, you can use regex

>>> import re
>>> re.search('^\s*[0-9]',"0abc")
<_sre.SRE_Match object at 0xb7722fa8>

answered Apr 7, 2011 at 7:38

kurumi

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2 Comments

Jakob Bowyer Over a year ago

overkill! Why use re when you can do it in the bultins

jeromej Over a year ago

@JakobBowyer At least, it is good to know about it! Even though in this case, yes, regexes ARE overkill if some builtins can do the same.

John Machin · Accepted Answer · 2011-04-07 09:49:50Z

10

Your code won't work; you need or instead of ||.

Try

'0' <= strg[:1] <= '9'

or

strg[:1] in '0123456789'

or, if you are really crazy about startswith,

strg.startswith(('0', '1', '2', '3', '4', '5', '6', '7', '8', '9'))

edited Apr 7, 2011 at 9:49

answered Apr 7, 2011 at 9:35

John Machin

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5 Comments

eyquem Over a year ago

I would have upvoted you if there was only the solution with startswith(a tuple) . But the solutions with strg[:1] have two inconveniences: they don't raise an error if s may happen to be a list of strings, and they produce True if the string begins with SEVERAL digits.

Rob Grant Over a year ago

@eyquem they produce True if the string begins with SEVERAL digits So? Other answers don't check for that, because it wasn't a requirement that it had to check that other chars weren't numeric.

james-see Over a year ago

I am crazy for startswith and did implement it this way in my code. Thank you.

Jérôme Over a year ago

One more startswith possibility... startswith(tuple(str(i) for i in range(10)))

PascalVKooten Over a year ago

.isdigit() is preferred

Jaroslav Bezděk · Accepted Answer · 2019-11-15 15:00:36Z

5

This piece of code:

for s in ("fukushima", "123 is a number", ""):
    print s.ljust(20),  s[0].isdigit() if s else False

prints out the following:

fukushima            False
123 is a number      True
                     False

edited Nov 15, 2019 at 15:00

Jaroslav Bezděk

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answered May 6, 2011 at 19:07

eyquem

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2 Comments

John Machin Over a year ago

That is ugly. Use s[:1].isdigit() or s and s[0].isdigit()

eyquem Over a year ago

@John Machin What do you mean by 'ugly' ? Not readable ? I don't find your propositions here above more readable. s and s[0].isdigit() is even less readable in my opinion. Personally, I like the construct ... if ... else ... Taking account of your remark, I would just improve it like that: s[0].isdigit() if s!="" else False

One Face · Accepted Answer · 2017-07-26 16:42:47Z

3

You can also use try...except:

try:
    int(string[0])
    # do your stuff
except:
    pass # or do your stuff

answered Jul 26, 2017 at 16:42

One Face

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Comments

ramiro · Accepted Answer · 2022-02-24 13:24:39Z

3

Using the built-in string module:

>>> import string
>>> '30 or older'.startswith(tuple(string.digits))

The accepted answer works well for single strings. I needed a way that works with pandas.Series.str.contains. Arguably more readable than using a regular expression and a good use of a module that doesn't seem to be well-known.

edited Feb 24, 2022 at 13:24

answered Feb 24, 2022 at 13:09

ramiro

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Comments

score 2 · Accepted Answer · 2011-04-07 19:00:17Z

2

Here are my "answers" (trying to be unique here, I don't actually recommend either for this particular case :-)

Using ~~ord() and~~ the special a <= b <= c form:

//starts_with_digit = ord('0') <= ord(mystring[0]) <= ord('9')
//I was thinking too much in C. Strings are perfectly comparable.
starts_with_digit = '0' <= mystring[0] <= '9'

(This a <= b <= c, like a < b < c, is a special Python construct and it's kind of neat: compare 1 < 2 < 3 (true) and 1 < 3 < 2 (false) and (1 < 3) < 2 (true). This isn't how it works in most other languages.)

Using a regular expression:

import re
//starts_with_digit = re.match(r"^\d", mystring) is not None
//re.match is already anchored
starts_with_digit = re.match(r"\d", mystring) is not None

edited Apr 7, 2011 at 19:00

answered Apr 7, 2011 at 7:41

user166390

1 Comment

John Machin Over a year ago

(1) Lose the ord() (2) Lose the ^ in the regex.

Tovi7 · Accepted Answer · 2011-04-07 07:43:20Z

0

You could use regular expressions.

You can detect digits using:

if(re.search([0-9], yourstring[:1])):
#do something

The [0-9] par matches any digit, and yourstring[:1] matches the first character of your string

answered Apr 7, 2011 at 7:43

Tovi7

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Comments

Ilya Smagin · Accepted Answer · 2011-04-07 07:39:17Z

-3

Use Regular Expressions, if you are going to somehow extend method's functionality.

answered Apr 7, 2011 at 7:39

Ilya Smagin

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Comments

bradley.ayers · Accepted Answer · 2011-04-07 07:50:18Z

-5

Try this:

if string[0] in range(10):

edited Apr 7, 2011 at 7:50

answered Apr 7, 2011 at 7:35

bradley.ayers

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2 Comments

Petri Lehtinen Over a year ago

This doesn't work at all. range(10) produces a list of integers, while string[0] is a string of length 1.

phynfo Over a year ago

That doesnt work; the types dont match. But: if string[0] in map(str,range(10)) works

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