Regex pattern bug in _default_cast

[mpertierra]

@goodmami The code for the function is shown below.

def _default_cast(x):
    if isinstance(x, basestring):
        if x.startswith('"'):
            x = x  # strip quotes?
        elif re.match(
                r'-?(0|[1-9]\d*)(\.\d+[eE][-+]?|\.|[eE][-+]?)\d+', x):
            x = float(x)
        elif re.match(r'-?\d+', x):
            x = int(x)
    return x

Here are some examples that cause unexpected behavior.

_default_cast("123a") # expected: "123a", actual: ValueError from int()
_default_cast("-3.14z") # expected: "-3.14z", actual: ValueError from float()

The problem arises from using regex patterns without ^ and $. Adding these characters as shown below fixes the problems.

def _default_cast(x):
    if isinstance(x, basestring):
        if x.startswith('"'):
            x = x  # strip quotes?
        elif re.match(
                r'^-?(0|[1-9]\d*)(\.\d+[eE][-+]?|\.|[eE][-+]?)\d+$', x):
            x = float(x)
        elif re.match(r'^-?\d+$', x):
            x = int(x)
    return x

Another alternative would be not to use regex at all here, and use try-except blocks instead, as shown below.

def _default_cast(x):
    if isinstance(x, basestring):
        try:
            x = int(x)
        except ValueError:
            try:
                x = float(x)
            except ValueError:
                pass
    return x

[goodmami]

Thanks! I've confirmed the problem.

Since I'm using re.match() and not re.search(), the initial ^ anchor is unnecessary, but you're right that the final $ is important.

I added some unit tests to model the issue, then tried both of your solutions. They're both equally capable, but when I benchmarked them the latter solution is a bit slower (except when the value is an integer), so I think I'll go with the first one.