Data Structures
Since Python is a dynamically typed language, Python values, not variables, carry type. This has implications for many aspects of the way the language functions.
All variables in Python hold references to objects, and these references are passed to functions; a function cannot change the value of variable references in its calling function. Some people (including Guido van Rossum himself) have called this parameter-passing scheme "Call by object reference."
Among dynamically typed languages, Python is moderately type-checked. Implicit conversion is defined for numeric types (as well as booleans), so one may validly multiply a complex number by a long integer (for instance) without explicit casting. However, there is no implicit conversion between (e.g.) numbers and strings; a string is an invalid argument to a mathematical function expecting a number.
Read more about this topic: Python Syntax And Semantics
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