Программа не работает. Что делать?
Моя программа не работает! Что делать? В данной статье я постараюсь собрать наиболее частые ошибки начинающих программировать на python 3, а также расскажу, как их исправлять.
Проблема: Моя программа не запускается. На доли секунды появляется чёрное окошко, а затем исчезает.
Причина: после окончания выполнения программы (после выполнения всего кода или при возникновении исключения программа закрывается. И если вы её вызвали двойным кликом по иконке (а вы, скорее всего, вызвали её именно так), то она закроется вместе с окошком, в котором находится вывод программы.
Решение: запускать программу через IDLE или через консоль.
Проблема: Не работает функция input. Пишет SyntaxError.
Причина: Вы запустили Python 2.
Проблема: Где-то увидел простую программу, а она не работает.
Причина: Вам подсунули программу на Python 2.
Решение: Прочитать об отличиях Python 2 от Python 3. Переписать её на Python 3. Например, данная программа на Python 3 будет выглядеть так:
Проблема: TypeError: Can’t convert ‘int’ object to str implicitly.
Причина: Нельзя складывать строку с числом.
Решение: Привести строку к числу с помощью функции int(). Кстати, заметьте, что функция input() всегда возвращает строку!
Проблема: SyntaxError: invalid syntax.
Причина: Забыто двоеточие.
Проблема: SyntaxError: invalid syntax.
Причина: Забыто равно.
Проблема: NameError: name ‘a’ is not defined.
Причина: Переменная «a» не существует. Возможно, вы опечатались в названии или забыли инициализировать её.
Решение: Исправить опечатку.
Проблема: IndentationError: expected an indented block.
Причина: Нужен отступ.
Проблема: TabError: inconsistent use of tabs and spaces in indentation.
Причина: Смешение пробелов и табуляции в отступах.
Решение: Исправить отступы.
Проблема: UnboundLocalError: local variable ‘a’ referenced before assignment.
Причина: Попытка обратиться к локальной переменной, которая ещё не создана.
Проблема: Программа выполнилась, но в файл ничего не записалось / записалось не всё.
Причина: Не закрыт файл, часть данных могла остаться в буфере.
Проблема: Здесь может быть ваша проблема. Комментарии чуть ниже 🙂
Как чинить SyntaxError
SyntaxError — это ошибка, которая легко может ввести в ступор начинающего программиста. Стоит забыть одну запятую или не там поставить кавычку и Python наотрез откажется запускать программу. Что ещё хуже, по выводу в консоль сложно сообразить в чём дело. Выглядят сообщения страшно и непонятно. Что с этим делать — не ясно. Вот неполный список того, что можно встретить:
Работать будем с программой, которая выводит на экран список учеников. Её код выглядит немного громоздко и, возможно, непривычно. Если не всё написанное вам понятно, то не отчаивайтесь, чтению статьи это не помешает.
Ожидается примерно такой результат в консоли:
Но запуск программы приводит к совсем другому результату. Скрипт сломан:
Первое слово SyntaxError Яндекс не понял. Помогите ему и разделите слова пробелом:
Теория. Синтаксические ошибки
Программирование — это не магия, а Python — не волшебный шар. Он не умеет предсказывать будущее, у него нет доступа к секретным знаниями, это просто автомат, это программа. Узнайте как она работает, как ищет ошибки в коде, и тогда легко найдете эффективный способ отладки. Вся необходимая теория собрана в этом разделе, дочитайте до конца.
SyntaxError — это синтаксическая ошибка. Она случается очень рано, еще до того, как Python запустит программу. Вот что делает компьютер, когда вы запускаете скрипт командой python script.py :
Синтаксическая ошибка SyntaxError возникает на четвёртом этапе в момент, когда Python разбирает текст программы на понятные ему компоненты. Сложные выражения в коде он разбирает на простейшие инструкции. Вот пример кода и инструкции для него:
SyntaxError случается когда Python не смог разбить сложный код на простые инструкции. Зная это, вы можете вручную разбить код на инструкции, чтобы затем проверить каждую из них по отдельности. Ошибка прячется в одной из инструкций.
1. Найдите поломанное выражение
Этот шаг сэкономит вам кучу сил. Найдите в программе сломанный участок кода. Его вам предстоит разобрать на отдельные инструкции. Посмотрите на вывод программы в консоль:
Вторая строчка сообщает: File «script.py», line 9 — ошибка в файле script.py на девятой строчке. Но эта строка является частью более сложного выражения, посмотрите на него целиком:
2. Разбейте выражение на инструкции
В прошлых шагах вы узнали что сломан этот фрагмент кода:
Разберите его на инструкции:
Так выделил бы инструкции программист, но вот Python сделать так не смог и сломался. Пора выяснить на какой инструкции нашла коса на камень.
Теперь ваша задача переписать код так, чтобы в каждой строке программы исполнялось не более одной инструкции из списка выше. Так вы сможете тестировать их по отдельности и облегчите себе задачу. Так выглядит отделение инструкции по созданию строки:
Сразу запустите код, проверьте что ошибка осталась на прежнему месте. Приступайте ко второй инструкции:
Скорее всего, Python не распознал вызов функции. Проверьте это, избавьтесь от последней инструкции — от создания переменной label :
3. Проверьте синтаксис вызова функции
Теперь вы знаете что проблема в коде, вызывающем функцию. Можно помедитировать еще немного над кодом программы, пройтись по нему зорким взглядом еще разок в надежде на лучшее. А можно поискать в сети примеры кода для сравнения.
Запросите у Яндекса статьи по фразе “Python синтаксис функции”, а в них поищите код, похожий на вызов format и сравните. Вот одна из первых статей в поисковой выдаче:
Уверен, теперь вы нашли ошибку. Победа!
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Invalid Syntax in Python: Common Reasons for SyntaxError
Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Identify Invalid Python Syntax
Python is known for its simple syntax. However, when you’re learning Python for the first time or when you’ve come to Python with a solid background in another programming language, you may run into some things that Python doesn’t allow. If you’ve ever received a SyntaxError when trying to run your Python code, then this guide can help you. Throughout this tutorial, you’ll see common examples of invalid syntax in Python and learn how to resolve the issue.
By the end of this tutorial, you’ll be able to:
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Invalid Syntax in Python
When you run your Python code, the interpreter will first parse it to convert it into Python byte code, which it will then execute. The interpreter will find any invalid syntax in Python during this first stage of program execution, also known as the parsing stage. If the interpreter can’t parse your Python code successfully, then this means that you used invalid syntax somewhere in your code. The interpreter will attempt to show you where that error occurred.
SyntaxError Exception and Traceback
When the interpreter encounters invalid syntax in Python code, it will raise a SyntaxError exception and provide a traceback with some helpful information to help you debug the error. Here’s some code that contains invalid syntax in Python:
Note that the traceback message locates the error in line 5, not line 4. The Python interpreter is attempting to point out where the invalid syntax is. However, it can only really point to where it first noticed a problem. When you get a SyntaxError traceback and the code that the traceback is pointing to looks fine, then you’ll want to start moving backward through the code until you can determine what’s wrong.
In the example above, there isn’t a problem with leaving out a comma, depending on what comes after it. For example, there’s no problem with a missing comma after ‘michael’ in line 5. But once the interpreter encounters something that doesn’t make sense, it can only point you to the first thing it found that it couldn’t understand.
Note: This tutorial assumes that you know the basics of Python’s tracebacks. To learn more about the Python traceback and how to read them, check out Understanding the Python Traceback and Getting the Most out of a Python Traceback.
There are a few elements of a SyntaxError traceback that can help you determine where the invalid syntax is in your code:
There are two other exceptions that you might see Python raise. These are equivalent to SyntaxError but have different names:
These exceptions both inherit from the SyntaxError class, but they’re special cases where indentation is concerned. An IndentationError is raised when the indentation levels of your code don’t match up. A TabError is raised when your code uses both tabs and spaces in the same file. You’ll take a closer look at these exceptions in a later section.
Common Syntax Problems
When you encounter a SyntaxError for the first time, it’s helpful to know why there was a problem and what you might do to fix the invalid syntax in your Python code. In the sections below, you’ll see some of the more common reasons that a SyntaxError might be raised and how you can fix them.
Misusing the Assignment Operator ( = )
There are several cases in Python where you’re not able to make assignments to objects. Some examples are assigning to literals and function calls. In the code block below, you can see a few examples that attempt to do this and the resulting SyntaxError tracebacks:
The first example tries to assign the value 5 to the len() call. The SyntaxError message is very helpful in this case. It tells you that you can’t assign a value to a function call.
The second and third examples try to assign a string and an integer to literals. The same rule is true for other literal values. Once again, the traceback messages indicate that the problem occurs when you attempt to assign a value to a literal.
Note: The examples above are missing the repeated code line and caret ( ^ ) pointing to the problem in the traceback. The exception and traceback you see will be different when you’re in the REPL vs trying to execute this code from a file. If this code were in a file, then you’d get the repeated code line and caret pointing to the problem, as you saw in other cases throughout this tutorial.
It’s likely that your intent isn’t to assign a value to a literal or a function call. For instance, this can occur if you accidentally leave off the extra equals sign ( = ), which would turn the assignment into a comparison. A comparison, as you can see below, would be valid:
Most of the time, when Python tells you that you’re making an assignment to something that can’t be assigned to, you first might want to check to make sure that the statement shouldn’t be a Boolean expression instead. You may also run into this issue when you’re trying to assign a value to a Python keyword, which you’ll cover in the next section.
Misspelling, Missing, or Misusing Python Keywords
Python keywords are a set of protected words that have special meaning in Python. These are words you can’t use as identifiers, variables, or function names in your code. They’re a part of the language and can only be used in the context that Python allows.
There are three common ways that you can mistakenly use keywords:
Another common issue with keywords is when you miss them altogether:
Once again, the exception message isn’t that helpful, but the traceback does attempt to point you in the right direction. If you move back from the caret, then you can see that the in keyword is missing from the for loop syntax.
You can also misuse a protected Python keyword. Remember, keywords are only allowed to be used in specific situations. If you use them incorrectly, then you’ll have invalid syntax in your Python code. A common example of this is the use of continue or break outside of a loop. This can easily happen during development when you’re implementing things and happen to move logic outside of a loop:
Here, Python does a great job of telling you exactly what’s wrong. The messages «‘break’ outside loop» and «‘continue’ not properly in loop» help you figure out exactly what to do. If this code were in a file, then Python would also have the caret pointing right to the misused keyword.
Another example is if you attempt to assign a Python keyword to a variable or use a keyword to define a function:
The list of protected keywords has changed with each new version of Python. For example, in Python 3.6 you could use await as a variable name or function name, but as of Python 3.7, that word has been added to the keyword list. Now, if you try to use await as a variable or function name, this will cause a SyntaxError if your code is for Python 3.7 or later.
| Version | print Type | Takes A Value |
|---|---|---|
| Python 2 | keyword | no |
| Python 3 | built-in function | yes |
print is a keyword in Python 2, so you can’t assign a value to it. In Python 3, however, it’s a built-in function that can be assigned values.
You can run the following code to see the list of keywords in whatever version of Python you’re running:
This code will tell you quickly if the identifier that you’re trying to use is a keyword or not.
Missing Parentheses, Brackets, and Quotes
Often, the cause of invalid syntax in Python code is a missed or mismatched closing parenthesis, bracket, or quote. These can be hard to spot in very long lines of nested parentheses or longer multi-line blocks. You can spot mismatched or missing quotes with the help of Python’s tracebacks:
Here, the traceback points to the invalid code where there’s a t’ after a closing single quote. To fix this, you can make one of two changes:
Another common mistake is to forget to close string. With both double-quoted and single-quoted strings, the situation and traceback are the same:
Quotes missing from statements inside an f-string can also lead to invalid syntax in Python:
Here, the reference to the ages dictionary inside the printed f-string is missing the closing double quote from the key reference. The resulting traceback is as follows:
Python identifies the problem and tells you that it exists inside the f-string. The message «unterminated string» also indicates what the problem is. The caret in this case only points to the beginning of the f-string.
This might not be as helpful as when the caret points to the problem area of the f-string, but it does narrow down where you need to look. There’s an unterminated string somewhere inside that f-string. You just have to find out where. To fix this problem, make sure that all internal f-string quotes and brackets are present.
The situation is mostly the same for missing parentheses and brackets. If you leave out the closing square bracket from a list, for example, then Python will spot that and point it out. There are a few variations of this, however. The first is to leave the closing bracket off of the list:
When you run this code, you’ll be told that there’s a problem with the call to print() :
Another variation is to add a trailing comma after the last element in the list while still leaving off the closing square bracket:
Now you get a different traceback:
In the previous example, 3 and print(foo()) were lumped together as one element, but here you see a comma separating the two. Now, the call to print(foo()) gets added as the fourth element of the list, and Python reaches the end of the file without the closing bracket. The traceback tells you that Python got to the end of the file (EOF), but it was expecting something else.
In this example, Python was expecting a closing bracket ( ] ), but the repeated line and caret are not very helpful. Missing parentheses and brackets are tough for Python to identify. Sometimes the only thing you can do is start from the caret and move backward until you can identify what’s missing or wrong.
Mistaking Dictionary Syntax
You saw earlier that you could get a SyntaxError if you leave the comma off of a dictionary element. Another form of invalid syntax with Python dictionaries is the use of the equals sign ( = ) to separate keys and values, instead of the colon:
Once again, this error message is not very helpful. The repeated line and caret, however, are very helpful! They’re pointing right to the problem character.
This type of issue is common if you confuse Python syntax with that of other programming languages. You’ll also see this if you confuse the act of defining a dictionary with a dict() call. To fix this, you could replace the equals sign with a colon. You can also switch to using dict() :
You can use dict() to define the dictionary if that syntax is more helpful.
Using the Wrong Indentation
There are two sub-classes of SyntaxError that deal with indentation issues specifically:
While other programming languages use curly braces to denote blocks of code, Python uses whitespace. That means that Python expects the whitespace in your code to behave predictably. It will raise an IndentationError if there’s a line in a code block that has the wrong number of spaces:
This might be tough to see, but line 5 is only indented 2 spaces. It should be in line with the for loop statement, which is 4 spaces over. Thankfully, Python can spot this easily and will quickly tell you what the issue is.
There’s also a bit of ambiguity here, though. Is the print(‘done’) line intended to be after the for loop or inside the for loop block? When you run the above code, you’ll see the following error:
If your tab size is the same width as the number of spaces in each indentation level, then it might look like all the lines are at the same level. However, if one line is indented using spaces and the other is indented with tabs, then Python will point this out as a problem:
Here, line 5 is indented with a tab instead of 4 spaces. This code block could look perfectly fine to you, or it could look completely wrong, depending on your system settings.
Python, however, will notice the issue immediately. But before you run the code to see what Python will tell you is wrong, it might be helpful for you to see an example of what the code looks like under different tab width settings:
Notice the difference in display between the three examples above. Most of the code uses 4 spaces for each indentation level, but line 5 uses a single tab in all three examples. The width of the tab changes, based on the tab width setting:
When you run the code, you’ll get the following error and traceback:
The solution to this is to make all lines in the same Python code file use either tabs or spaces, but not both. For the code blocks above, the fix would be to remove the tab and replace it with 4 spaces, which will print ‘done’ after the for loop has finished.
Defining and Calling Functions
You might run into invalid syntax in Python when you’re defining or calling functions. For example, you’ll see a SyntaxError if you use a semicolon instead of a colon at the end of a function definition:
The traceback here is very helpful, with the caret pointing right to the problem character. You can clear up this invalid syntax in Python by switching out the semicolon for a colon.
In addition, keyword arguments in both function definitions and function calls need to be in the right order. Keyword arguments always come after positional arguments. Failure to use this ordering will lead to a SyntaxError :
Here, once again, the error message is very helpful in telling you exactly what is wrong with the line.
Changing Python Versions
Sometimes, code that works perfectly fine in one version of Python breaks in a newer version. This is due to official changes in language syntax. The most well-known example of this is the print statement, which went from a keyword in Python 2 to a built-in function in Python 3:
This is one of the examples where the error message provided with the SyntaxError shines! Not only does it tell you that you’re missing parenthesis in the print call, but it also provides the correct code to help you fix the statement.
Another problem you might encounter is when you’re reading or learning about syntax that’s valid syntax in a newer version of Python, but isn’t valid in the version you’re writing in. An example of this is the f-string syntax, which doesn’t exist in Python versions before 3.6:
In versions of Python before 3.6, the interpreter doesn’t know anything about the f-string syntax and will just provide a generic «invalid syntax» message. The problem, in this case, is that the code looks perfectly fine, but it was run with an older version of Python. When in doubt, double-check which version of Python you’re running!
Python syntax is continuing to evolve, and there are some cool new features introduced in Python 3.8:
This TypeError means that you can’t call a tuple like a function, which is what the Python interpreter thinks you’re doing.
The helpful message accompanying the new SyntaxWarning even provides a hint ( «perhaps you missed a comma?» ) to point you in the right direction!
Conclusion
In this tutorial, you’ve seen what information the SyntaxError traceback gives you. You’ve also seen many common examples of invalid syntax in Python and what the solutions are to those problems. Not only will this speed up your workflow, but it will also make you a more helpful code reviewer!
When you’re writing code, try to use an IDE that understands Python syntax and provides feedback. If you put many of the invalid Python code examples from this tutorial into a good IDE, then they should highlight the problem lines before you even get to execute your code.
Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Identify Invalid Python Syntax
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About Chad Hansen
Chad is an avid Pythonista and does web development with Django fulltime. Chad lives in Utah with his wife and six kids.
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