What's the difference between str.isdigit, isnumeric and isdecimal in python?

It's mostly about unicode classifications. Here's some examples to show discrepancies:

>>> def spam(s):
...     for attr in 'isnumeric', 'isdecimal', 'isdigit':
...         print(attr, getattr(s, attr)())
...         
>>> spam('½')
isnumeric True
isdecimal False
isdigit False
>>> spam('³')
isnumeric True
isdecimal False
isdigit True

Specific behaviour is in the official docs here.

Script to find all of them:

import sys
import unicodedata
from collections import defaultdict

d = defaultdict(list)
for i in range(sys.maxunicode + 1):
    s = chr(i)
    t = s.isnumeric(), s.isdecimal(), s.isdigit()
    if len(set(t)) == 2:
        try:
            name = unicodedata.name(s)
        except ValueError:
            name = f'codepoint{i}'
        print(s, name)
        d[t].append(s)

By definition, isdecimal() ⊆ isdigit() ⊆ isnumeric(). That is, if a string is decimal, then it'll also be digit and numeric.

Therefore, given a string s and test it with those three methods, there'll only be 4 types of results.

+-------------+-----------+-------------+----------------------------------+
| isdecimal() | isdigit() | isnumeric() |          Example                 |
+-------------+-----------+-------------+----------------------------------+
|    True     |    True   |    True     | "038", "੦੩੮", "０３８"           |
|  False      |    True   |    True     | "⁰³⁸", "⒊⒏", "⓪③⑧"          |
|  False      |  False    |    True     | "↉⅛⅘", "ⅠⅢⅧ", "⑩⑬㊿", "壹貳參"  |
|  False      |  False    |  False      | "abc", "38.0", "-38"             |
+-------------+-----------+-------------+----------------------------------+

---

1. Some examples of characters isdecimal()==True

(thus isdigit()==True and isnumeric()==True)

"0123456789"  DIGIT ZERO~NINE
"٠١٢٣٤٥٦٧٨٩"  ARABIC-INDIC DIGIT ZERO~NINE
"०१२३४५६७८९"  DEVANAGARI DIGIT ZERO~NINE
"০১২৩৪৫৬৭৮৯"  BENGALI DIGIT ZERO~NINE
"੦੧੨੩੪੫੬੭੮੯"  GURMUKHI DIGIT ZERO~NINE
"૦૧૨૩૪૫૬૭૮૯"  GUJARATI DIGIT ZERO~NINE
"୦୧୨୩୪୫୬୭୮୯"  ORIYA DIGIT ZERO~NINE
"௦௧௨௩௪௫௬௭௮௯"  TAMIL DIGIT ZERO~NINE
"౦౧౨౩౪౫౬౭౮౯"  TELUGU DIGIT ZERO~NINE
"೦೧೨೩೪೫೬೭೮೯"  KANNADA DIGIT ZERO~NINE
"൦൧൨൩൪൫൬൭൮൯"  MALAYALAM DIGIT ZERO~NINE
"๐๑๒๓๔๕๖๗๘๙"  THAI DIGIT ZERO~NINE
"໐໑໒໓໔໕໖໗໘໙"  LAO DIGIT ZERO~NINE
"༠༡༢༣༤༥༦༧༨༩"  TIBETAN DIGIT ZERO~NINE
"၀၁၂၃၄၅၆၇၈၉"  MYANMAR DIGIT ZERO~NINE
"០១២៣៤៥៦៧៨៩"  KHMER DIGIT ZERO~NINE
"０１２３４５６７８９"  FULLWIDTH DIGIT ZERO~NINE
""  MATHEMATICAL BOLD DIGIT ZERO~NINE
""  MATHEMATICAL DOUBLE-STRUCK DIGIT ZERO~NINE
""  MATHEMATICAL SANS-SERIF DIGIT ZERO~NINE
""  MATHEMATICAL SANS-SERIF BOLD DIGIT ZERO~NINE
""  MATHEMATICAL MONOSPACE DIGIT ZERO~NINE

2. Some examples of characters isdecimal()==False but isdigit()==True

(thus isnumeric()==True)

"⁰¹²³⁴⁵⁶⁷⁸⁹"  SUPERSCRIPT ZERO~NINE
"₀₁₂₃₄₅₆₇₈₉"  SUBSCRIPT ZERO~NINE
"⒈⒉⒊⒋⒌⒍⒎⒏⒐"  DIGIT ZERO~NINE FULL STOP
""  DIGIT ZERO~NINE COMMA
"⓪①②③④⑤⑥⑦⑧⑨"  CIRCLED DIGIT ZERO~NINE
"⓿❶❷❸❹❺❻❼❽❾"  NEGATIVE CIRCLED DIGIT ZERO~NINE
"⑴⑵⑶⑷⑸⑹⑺⑻⑼"  PARENTHESIZED DIGIT ONE~NINE
"➀➁➂➃➄➅➆➇➈"  DINGBAT CIRCLED SANS-SERIF DIGIT ONE~NINE
"⓵⓶⓷⓸⓹⓺⓻⓼⓽"  DOUBLE CIRCLED DIGIT ONE~NINE
"➊➋➌➍➎➏➐➑➒"  DINGBAT NEGATIVE CIRCLED SANS-SERIF DIGIT ONE~NINE
"፩፪፫፬፭፮፯፰፱"  ETHIOPIC DIGIT ONE~NINE

3. Some examples of characters isdecimal()==False and isdigit()==False but isnumeric()==True

"½⅓¼⅕⅙⅐⅛⅑⅒⅔¾⅖⅗⅘⅚⅜⅝⅞⅟↉"  VULGAR FRACTION
"৴৵৶৷৸৹"  BENGALI CURRENCY NUMERATOR
"௰௱௲"  TAMIL NUMBER TEN, ONE HUNDRED, ONE THOUSAND
"౸౹౺౻౼౽౾"  TELUGU FRACTION DIGIT
"൰൱൲൳൴൵"  MALAYALAM NUMBER, MALAYALAM FRACTION
"༳༪༫༬༭༮༯༰༱༲"  TIBETAN DIGIT HALF ZERO~NINE
"፲፳፴፵፶፷፸፹፺፻፼"  ETHIOPIC NUMBER TEN~NINETY, HUNDRED, TEN THOUSAND
"៰៱៲៳៴៵៶៷៸៹"  KHMER SYMBOL LEK ATTAK
"ⅠⅡⅢⅣⅤⅥⅦⅧⅨⅩⅪⅫⅬⅭⅮⅯ"  ROMAN NUMERAL
"ⅰⅱⅲⅳⅴⅵⅶⅷⅸⅹⅺⅻⅼⅽⅾⅿ"  SMALL ROMAN NUMERAL
"ↀↁↂↅↆ"  ROMAN NUMERAL
"⑩⑪⑫⑬⑭⑮⑯⑰⑱⑲⑳㉑㉒㉓㉔㉕㉖㉗㉘㉙㉚㉛㉜㉝㉞㉟㊱㊲㊳㊴㊵㊶㊷㊸㊹㊺㊻㊼㊽㊾㊿"  CIRCLED NUMBER TEN~FIFTY
"㉈㉉㉊㉋㉌㉍㉎㉏"  CIRCLED NUMBER TEN~EIGHTY ON BLACK SQUARE
"⑽⑾⑿⒀⒁⒂⒃⒄⒅⒆⒇"  PARENTHESIZED NUMBER TEN~TWENTY
"⒑⒒⒓⒔⒕⒖⒗⒘⒙⒚⒛"  NUMBER TEN~TWENTY FULL STOP
"⓫⓬⓭⓮⓯⓰⓱⓲⓳⓴"  NEGATIVE CIRCLED NUMBER ELEVEN
"⓾➉❿➓"  various styles of CIRCLED NUMBER TEN
""  DINGBAT NEGATIVE CIRCLED SANS-SERIF DIGIT ZERO
"〇"  IDEOGRAPHIC NUMBER ZERO
"〡〢〣〤〥〦〧〨〩〸〹〺"  HANGZHOU NUMERAL ONE~TEN, TWENTY, THIRTY
"㆒㆓㆔㆕"  IDEOGRAPHIC ANNOTATION ONE~FOUR MARK
"㈠㈡㈢㈣㈤㈥㈦㈧㈨㈩"  PARENTHESIZED IDEOGRAPH ONE~TEN
"㊀㊁㊂㊃㊄㊅㊆㊇㊈㊉"  CIRCLED IDEOGRAPH ONE~TEN
"一二三四五六七八九十壹貳參肆伍陸柒捌玖拾零百千萬億兆弐貮贰㒃㭍漆什㐅陌阡佰仟万亿幺兩㠪亖卄卅卌廾廿"  CJK UNIFIED IDEOGRAPH
"參拾兩零六陸什"  CJK COMPATIBILITY IDEOGRAPH
""  AEGEAN NUMBER ONE~NINE, TEN~NINETY
""  AEGEAN NUMBER ONE~NINE HUNDRED, ONE~NINE THOUSAND
""  AEGEAN NUMBER TEN~NINETY THOUSAND
""  GREEK ACROPHONIC ATTIC
""  COUNTING ROD UNIT DIGIT ONE~NINE
""  COUNTING ROD TENS DIGIT ONE~NINE

The Python documentation notes the difference between the three methods.

str.isdigit

Return true if all characters in the string are digits and there is at least one character, false otherwise. Digits include decimal characters and digits that need special handling, such as the compatibility superscript digits. This covers digits which cannot be used to form numbers in base 10, like the Kharosthi numbers. Formally, a digit is a character that has the property value Numeric_Type=Digit or Numeric_Type=Decimal.

str.isnumeric

Return true if all characters in the string are numeric characters, and there is at least one character, false otherwise. Numeric characters include digit characters, and all characters that have the Unicode numeric value property, e.g. U+2155, VULGAR FRACTION ONE FIFTH. Formally, numeric characters are those with the property value Numeric_Type=Digit, Numeric_Type=Decimal or Numeric_Type=Numeric.

str.isdecimal

Return true if all characters in the string are decimal characters and there is at least one character, false otherwise. Decimal characters are those that can be used to form numbers in base 10, e.g. U+0660, ARABIC-INDIC DIGIT ZERO. Formally a decimal character is a character in the Unicode General Category “Nd”.

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Like @Wim said, the main difference between the three methods is the way they handle specific unicode characters.