Python String isidentifier()

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Checks whether all characters are identifiers that can be used as names of functions, classes, or variables (True or False).

Minimal Example

>>> 'var_1'.isidentifier()
True

As you read over the explanations below, feel free to watch our video guide about this particular string method:

Syntax and Explanation

str.isidentifier()

Checks whether all characters are identifiers that can be used as names of functions, classes, or variables (True or False).

Here’s the relevant part of the definition of a valid Python identifier:

Within the ASCII range (U+0001..U+007F), the valid characters for identifiers are the same as in Python 2.x: the uppercase and lowercase letters A through Z, the underscore _ and, except for the first character, the digits 0 through 9.

Examples of good identifiers:

  • my_var_1
  • yourvar
  • x_111
  • VARIABLE_XXX

Note that to test whether a given string is a reserved identifier, it uses the function keyword.iskeyword(). Here’s the list of Python keywords that are reserved and shouldn’t be used as identifiers:

False      await      else       import     pass
None       break      except     in         raise
True       class      finally    is         return
and        continue   for        lambda     try
as         def        from       nonlocal   while
assert     del        global     not        with
async      elif       if         or         yield
>>> 'hi'.isidentifier()
True
>>> 'def'.isidentifier()
True

So, you could take reserved keywords as identifiers although this would result in unreadable code. To check whether an identifier x is a reserved keyword, use the keyword.iskeyword(x) function:

>>> import keyword
>>> keyword.iskeyword('def')
True

More String Methods

Python’s string class comes with a number of useful additional string methods. Here’s a short collection of all Python string methods—each link opens a short tutorial in a new tab.

MethodDescription
capitalize()Return a copy of the string with capitalized first character and lowercased remaining characters.
casefold()Return a lowercased, casefolded string similar to lowercase() but more aggressive.
center()Return a centered string of a certain length, padded with whitespace or custom characters.
count()Return the number of non-overlapping occurrences of a substring.
encode()Returns a byte object that is an encoded version of the string.
endswith()Returns whether the string ends with a given value or not (True or False).
expandtabs()Return a string with spaces instead of tab characters.
find()Returns the index of the first occurrence of the specified substring.
format()Formats the string according to the Format Description Language.
format_map()Formats the string according to the Format Description Language, passing a mapping object.
index()Returns the index of the first occurrence of the specified substring, like find() but it raises a ValueError if the substring is not found.
isalnum()Checks whether all characters are alphabetic or numeric (True or False).
isalpha()Checks whether all characters are alphabetic (True or False).
isascii()Checks whether all characters are ASCII (True or False).
isdecimal()Checks whether all characters are decimal numbers (True or False).
isdigit()Checks whether all characters are digits, i.e., numbers from 0 to 9 (True or False).
isidentifier()Checks whether all characters are identifiers that can be used as names of functions, classes, or variables (True or False).
islower()Checks whether all characters are lowercase (True or False).
isnumeric()Checks whether all characters are numeric values (True or False).
isprintable()Checks whether all characters are printable (True or False).
isspace()Checks whether all characters are whitespaces (True or False).
istitle()Checks if the string is title-cased (True or False).
isupper()Checks whether all characters are uppercase (True or False).
join()Concatenates the elements in an iterable.
ljust()Returns a left-justified string filling up the right-hand side with fill characters.
lower()Returns a lowercase string version.
lstrip()Trims whitespaces on the left and returns a new string.
maketrans()Returns a translation table.
partition()Searches for a separator substring and returns a tuple with three strings: (1) everything before the separator, (2) the separator itself, and (3) everything after it.
removeprefix()Return string[len(prefix):] if the string starts with prefix, and string[:] otherwise.
removesuffix()Return string[:-len(suffix)] if the string starts with suffix, and string[:] otherwise.
replace()Returns a string with replaced values.
rfind()Return the highest index in the string where a substring is found. Returns -1 if not found.
rindex()Return the highest index in the string where a substring is found. Returns ValueError if not found.
rjust()Returns a right-justified string filling up the left-hand side with fill characters.
rpartition()Searches for a separator substring and returns a tuple with three strings: (1) everything before the separator, (2) the separator itself, and (3) everything after it.
rsplit()Splits the string at a given separator and returns a split list of substrings.
rstrip()Trims whitespaces on the right and returns a new string.
split()Splits the string at a given separator and returns a split list of substrings.
splitlines()Splits the string at line breaks such as '\n' and returns a split list of substrings (i.e., lines).
startswith()Returns whether the string starts with a given value or not (True or False).
strip()Trims whitespaces on the left and right and returns a new string.
swapcase()Swaps lowercase to uppercase characters and vice versa.
title()Returns a new string with uppercase first characters of each word.
translate()Returns a translated string.
upper()Returns a lowercase string version.
zfill()Fills the string from the left with "0" characters.

References