How do I parse a string to a float or int?

How can I convert an str to a float?

"545.2222" -> 545.2222

Or an str to a int?

"31" -> 31

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_{For the reverse, see Convert integer to string in Python and Converting a float to a string without rounding it.}

_{Please instead use How can I read inputs as numbers? to close duplicate questions where OP received a string from user input and immediately wants to convert it, or was hoping for input (in 3.x) to convert the type automatically.}