在 Python 中使用 format() 方法打印布尔值 True/False

2022-01-19 00:00:00 python python-2.7 string-formatting boolean string.format

问题描述

我试图打印布尔表达式的真值表.在执行此操作时，我偶然发现了以下内容:

I was trying to print a truth table for Boolean expressions. While doing this, I stumbled upon the following:

>>> format(True, "") # shows True in a string representation, same as str(True)
'True'
>>> format(True, "^") # centers True in the middle of the output string
'1'

只要我指定了格式说明符，format() 就会将 True 转换为 1.我知道 bool 是 int 的子类，因此 True 的计算结果为 1:

As soon as I specify a format specifier, format() converts True to 1. I know that bool is a subclass of int, so that True evaluates to 1:

>>> format(True, "d") # shows True in a decimal format
'1'

但是为什么在第一个示例中使用格式说明符会将 'True' 更改为 1?

But why does using the format specifier change 'True' to 1 in the first example?

我求助于文档进行说明.它唯一说的是:

I turned to the docs for clarification. The only thing it says is:

一般约定是空格式字符串 ("") 产生的结果与您对值调用 str() 产生的结果相同.非空格式字符串通常会修改结果.

A general convention is that an empty format string ("") produces the same result as if you had called str() on the value. A non-empty format string typically modifies the result.

因此，当您使用格式说明符时，字符串会被修改.但是，如果仅指定了对齐运算符(例如^)，为什么从True更改为1?p>

So the string gets modified when you use a format specifier. But why the change from True to 1 if only an alignment operator (e.g. ^) is specified?

解决方案

好问题！我相信我有答案.这需要在 C 语言中挖掘 Python 源代码，所以请耐心等待.

Excellent question! I believe I have the answer. This requires digging around through the Python source code in C, so bear with me.

首先，format(obj, format_spec) 只是 obj.__format__(format_spec) 的语法糖.对于发生这种情况的具体位置，您必须查看 abstract.c，在函数中:

First, format(obj, format_spec) is just syntactic sugar for obj.__format__(format_spec). For specifically where this occurs, you'd have to look in abstract.c, in the function:

PyObject *
PyObject_Format(PyObject* obj, PyObject *format_spec)
{
    PyObject *empty = NULL;
    PyObject *result = NULL;

    ...

    if (PyInstance_Check(obj)) {
        /* We're an instance of a classic class */
HERE -> PyObject *bound_method = PyObject_GetAttrString(obj, "__format__");
        if (bound_method != NULL) {
            result = PyObject_CallFunctionObjArgs(bound_method,
                                                  format_spec,
                                                  NULL);

    ...
}

要找到确切的调用，我们必须查看 intobject.c:

To find the exact call, we have to look in intobject.c:

static PyObject *
int__format__(PyObject *self, PyObject *args)
{
    PyObject *format_spec;

    ...

    return _PyInt_FormatAdvanced(self,
                     ^           PyBytes_AS_STRING(format_spec),
                     |           PyBytes_GET_SIZE(format_spec));
               LET'S FIND THIS
    ...
}

_PyInt_FormatAdvanced 实际上在 formatter_string.c 作为 formatter.h:

_PyInt_FormatAdvanced is actually defined as a macro in formatter_string.c as a function found in formatter.h:

static PyObject*
format_int_or_long(PyObject* obj,
               STRINGLIB_CHAR *format_spec,
           Py_ssize_t format_spec_len,
           IntOrLongToString tostring)
{
    PyObject *result = NULL;
    PyObject *tmp = NULL;
    InternalFormatSpec format;

    /* check for the special case of zero length format spec, make
       it equivalent to str(obj) */
    if (format_spec_len == 0) {
        result = STRINGLIB_TOSTR(obj);   <- EXPLICIT CAST ALERT!
        goto done;
    }

    ... // Otherwise, format the object as if it were an integer
}

这就是你的答案.简单检查 format_spec_len 是否为 0，如果是，将 obj 转换为字符串.众所周知，str(True)就是'True'，谜底结束！

And therein lies your answer. A simple check for whether format_spec_len is 0, and if it is, convert obj into a string. As you well know, str(True) is 'True', and the mystery is over!

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