“ with”语句中变量的作用域？

我只读取 python 的 firstline使用:

with open(file_path, 'r') as f:
my_count = f.readline()
print(my_count)

我对变量 my _ count 的作用域有点困惑。虽然打印工作良好，但是最好先在外部使用语句执行类似于 my_count = 0的操作(例如在 C 中用于执行 int my_count = 0)

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最佳答案

A with statement does not create a scope (like if, for and while do not create a scope either).

As a result, Python will analyze the code and see that you made an assignment in the with statement, and thus that will make the variable local (to the real scope).

In Python variables do not need initialization in all code paths: as a programmer, you are responsible to make sure that a variable is assigned before it is used. This can result in shorter code: say for instance you know for sure that a list contains at least one element, then you can assign in a for loop. In Java assignment in a for loop is not considered safe (since it is possible that the body of the loop is never executed).

Initialization before the with scope can be safer in the sense that after the with statement we can safely assume that the variable exists. If on the other hand the variable should be assigned in the with statement, not initializing it before the with statement actually results in an additional check: Python will error if somehow the assignment was skipped in the with statement.

A with statement is only used for context management purposes. It forces (by syntax) that the context you open in the with is closed at the end of the indentation.

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You should also go through PEP-343 and Python Documentation. It will clear that its not about creating scope its about using Context Manager. I am quoting python documentation on context manager

A context manager is an object that defines the runtime context to be established when executing a with statement. The context manager handles the entry into, and the exit from, the desired runtime context for the execution of the block of code. Context managers are normally invoked using the with statement (described in section The with statement), but can also be used by directly invoking their methods.

Typical uses of context managers include saving and restoring various kinds of global state, locking and unlocking resources, closing opened files, etc.


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