meaning of -> syntax in function definition

[DrakeSoft]

Hi,

I came across the following function:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

def getWords(self) -> list[str]:      with open(self.word_list_path, "r") as f:         word_list = [line.strip() for line in f]       return word_list   class Ui_MainWindow(QMainWindow):  def __init__(self, word_list_path: str):      super().__init__()      self.word_list_path = word_list_path      self.speller = SpellCheckWrapper(self.getWords(), self.addToDictionary)      self.setupUi()   class SpellCheckWrapper:     def __init__(         self, personal_word_list: list[str], addToDictionary: Callable[[str], None]     ):         # Creating temporary file         self.file = QTemporaryFile()         self.file.open()         self.dictionary = DictWithPWL(             "en_US",           .......

I am not familiar with the notation "def getWords(self) -> list[str]:" in the function definition. What does the -> mean?
Also when I invoke the file with python3 I get an error on the line:

1	self.speller = SpellCheckWrapper(self.getWords(), self.addToDictionary)

TypeError: 'type' object is not subscriptable

Could someone explain or provide some context for the use of -> and the error I see?

[deanhystad]

The "->" is part of the relatively new (version 3.5) Python type annotations. Think of type annotations as a way to document variable and function return types. An annotation standard allows development of code checking tools and documentation tools. Annotations do not change how Python runs the code. I would change "->list[str]" to "->List[str]", but this is not going to cause an errors.

Everything you 'd ever want to know about type annotations:
https://docs.python.org/3/library/typing.html
https://peps.python.org/pep-0484/
https://peps.python.org/pep-0483/

The annotations are not causing your error. I cannot help you because that is the only thing I know about your error. When you have an error please post the entire error message including the traceback. I would also like to see more complete code. This instruction is not correct for the code you have posed:

1	self.speller = SpellCheckWrapper(self.getWords(), self.addToDictionary)

According to this line, getWords() is a method of Ui_MainWindow, but it does not look like that in your posted code.

[Yoriz]

If you are using a version older than 3.9 you would need to import List from the typing module

1 2 3 4

from typing import List   def getWords(self) -> List[str]:     ....

---

From version python 3.9 you can use the built-in list as a type hint.

1 2	def getWords(self) -> list[str]:     ....

Type Hinting Generics in Standard Collections
In type annotations you can now use built-in collection types such as list and dict as generic types instead of importing the corresponding capitalized types (e.g. List or Dict) from typing. Some other types in the standard library are also now generic, for example queue.Queue.

Example:

1 2 3

def greet_all(names: list[str]) -> None:     for name in names:         print("Hello", name)

See PEP 585 for more details. (Contributed by Guido van Rossum, Ethan Smith, and Batuhan Taşkaya in bpo-39481.)

[DrakeSoft]

Thank you for your reply
I have included the complete code:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182

#example.py   import sys   from PyQt5.QtWidgets import QApplication, QMainWindow, QVBoxLayout, QWidget   from spellcheckwrapper import SpellCheckWrapper from spelltextedit import SpellTextEdit     class Ui_MainWindow(QMainWindow):     def __init__(self, word_list_path: str):         super().__init__()         self.word_list_path = word_list_path         self.speller = SpellCheckWrapper(self.getWords(), self.addToDictionary)         self.setupUi()       def setupUi(self):         self.resize(500, 500)         self.setWindowTitle("Example app")           self.centralWidget = QWidget(self)         self.setCentralWidget(self.centralWidget)           self.layout = QVBoxLayout(self.centralWidget)         self.centralWidget.setLayout(self.layout)           self.textEdit1 = SpellTextEdit(self.speller, self.centralWidget)         self.layout.addWidget(self.textEdit1)           self.textEdit2 = SpellTextEdit(self.speller, self.centralWidget)         self.layout.addWidget(self.textEdit2)       def getWords(self) -> list[str]:         with open(self.word_list_path, "r") as f:             word_list = [line.strip() for line in f]         return word_list       def addToDictionary(self, new_word: str):         with open(self.word_list_path, "a") as f:             f.write("\n" + new_word)     if __name__ == "__main__":     app = QApplication(sys.argv)     window = Ui_MainWindow("example_word_list.txt")     window.show()     sys.exit(app.exec_()) #------------------------   #spellcheckhighlighter.py   import re   from PyQt5.QtCore import Qt from PyQt5.QtGui import QSyntaxHighlighter, QTextCharFormat   from spellcheckwrapper import SpellCheckWrapper     class SpellCheckHighlighter(QSyntaxHighlighter):     wordRegEx = re.compile(r"\b([A-Za-z]{2,})\b")       def highlightBlock(self, text: str) -> None:         if not hasattr(self, "speller"):             return           self.misspelledFormat = QTextCharFormat()         self.misspelledFormat.setUnderlineStyle(QTextCharFormat.SpellCheckUnderline)         self.misspelledFormat.setUnderlineColor(Qt.red)           for word_object in self.wordRegEx.finditer(text):             if not self.speller.check(word_object.group()):                 self.setFormat(                     word_object.start(),                     word_object.end() - word_object.start(),                     self.misspelledFormat,                 )       def setSpeller(self, speller: SpellCheckWrapper):         self.speller = speller   #------------------------------------     from typing import Callable import enchant from enchant import DictWithPWL from PyQt5.QtCore import QTemporaryFile     class SpellCheckWrapper:     def __init__(         self, personal_word_list: list[str], addToDictionary: Callable[[str], None]     ):         # Creating temporary file         self.file = QTemporaryFile()         self.file.open()         self.dictionary = DictWithPWL(             "en_US",             self.file.fileName(),         )           self.addToDictionary = addToDictionary           self.word_list = set(personal_word_list)         self.load_words()       def load_words(self):         for word in self.word_list:             self.dictionary.add(word)       def suggestions(self, word: str) -> list[str]:         return self.dictionary.suggest(word)       def correction(self, word: str) -> str:         return self.dictionary.suggest(word)[0]       def add(self, new_word: str) -> bool:         if self.check(new_word):             return False         self.word_list.add(new_word)         self.addToDictionary(new_word)         self.dictionary.add(new_word)         return True       def check(self, word: str) -> bool:         return self.dictionary.check(word)       def getNewWords(self) -> set[str]:         return self.word_list   #--------------------------------------------   # highlighter.py   import re   from PyQt5.QtCore import Qt from PyQt5.QtGui import QSyntaxHighlighter, QTextCharFormat   from spellcheckwrapper import SpellCheckWrapper     class SpellCheckHighlighter(QSyntaxHighlighter):     wordRegEx = re.compile(r"\b([A-Za-z]{2,})\b")       def highlightBlock(self, text: str) -> None:         if not hasattr(self, "speller"):             return           self.misspelledFormat = QTextCharFormat()         self.misspelledFormat.setUnderlineStyle(QTextCharFormat.SpellCheckUnderline)         self.misspelledFormat.setUnderlineColor(Qt.red)           for word_object in self.wordRegEx.finditer(text):             if not self.speller.check(word_object.group()):                 self.setFormat(                     word_object.start(),                     word_object.end() - word_object.start(),                     self.misspelledFormat,                 )       def setSpeller(self, speller: SpellCheckWrapper):         self.speller = speller #   # correction_action.py from PyQt5.QtCore import pyqtSignal from PyQt5.QtWidgets import QAction     class SpecialAction(QAction):     actionTriggered = pyqtSignal(str)       def __init__(self, *args):         super().__init__(*args)           self.triggered.connect(self.emitTriggered)       def emitTriggered(self):         self.actionTriggered.emit(self.text())

The error message is

Error:
Traceback (most recent call last):
  File "/home/ProtoTyping/SpellChecker/pyqt-spellcheck/src/example.py", line 5, in <module>
    from spellcheckwrapper import SpellCheckWrapper
  File "/home/ProtoTyping/SpellChecker/pyqt-spellcheck/src/spellcheckwrapper.py", line 7, in <module>
    class SpellCheckWrapper:
  File "/home/ProtoTyping/SpellChecker/pyqt-spellcheck/src/spellcheckwrapper.py", line 9, in SpellCheckWrapper
    self, personal_word_list: list[str], addToDictionary: Callable[[str], None]
TypeError: 'type' object is not subscriptable

Yoriz write Apr-09-2022, 12:40 AM:
Please post all code, output and errors (In thier entirety) between their respective tags. Refer to BBCode help topic on how to post. Use the "Preview Post" button to make sure the code is presented as you expect before hitting the "Post Reply/Thread" button.

[deanhystad]

As Yoriz mentioned, if you are using Python 3.9 or newer you can use list[str]. If you are using an older version of Python you need to use List[str] and in spellecheckwrapper.py modify the import.

1	from typing import Callable, List

[DrakeSoft]

Thanks guys,

When I did a python --version it was showing me Python 3.9.7, so the original code should have worked. But I was running the code in VScode, so I guess it was using an earlier version.

In the settings of VSCode, I changed the interpreter from Python3 (which was defaulting to /usr/bin/python3 --version Python 3.8.10) to Python 3.9.7 and then the code ran fine in vscode also.

again, thank you for your help, very much appreciated!