Seite - 44 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Band Second Edition

44 2 AFewMoreSteps PythonisBoth DynamicallyandStronglyTyped Python is a dynamically typed language, since a certain variable name may refer to objects of different types during the execution of a program. This means that writing In [1]: z = 10 # z refers to an integer In [2]: z = 10.0 # z refers to a float In [3]: z = ’some string’ # z refers to a string is perfectly fine and we get no error messages. In statically typed languages (e.g.,C and Fortran), this would not be accepted, since a variable then would have tobe ofoneparticular type throughout. Python is also a strongly typed language, since it is strict about how you cancombineobject types: In [1]: ’yes’ + ’no’ # add two strings Out[1]: ’yesno’ In [2]: ’yes’ + 10 # ...try adding string and integer Traceback (most recent call last): File "<ipython-input-5-fdfd15f88bd0>", line 1, in <module> ’yes’ + 10 TypeError: cannot concatenate ’str’ and ’int’ objects Adding two strings is straight forward, but trying the same with a string and an integer gives an error message, since it is not well defined. In a weakly typed language, this could very well give yes10 without any error message. If so, it would be based on a (vaguely) founded assumption that this is what the programmer intended (but perhaps it is just as likely an error?). For more details on these matters, see, e.g., https://en.wikipedia.org/wiki/Python_(programming_language). 2.2.6 OperatorPrecedence When the arithmetic operators+, -, *, / and ** appear in an expression, Python gives thema certainprecedence.Python interprets the expressionfromleft to right, takingoneterm(partofexpressionbetweentwosuccessive+or-)ata time.Within each term,** isdonebefore*and/. Consider the expressionx = 1*5**2 + 10*3 - 1.0/4.Thereare three terms here and interpreting this, Python starts from the left. In the first term,1*5**2, it first does5**2which equals 25. This is then multiplied by 1 to give 25 again. The secondtermis10*3, i.e.,30.So thefirst twotermsaddupto55.Thelast termgives 0.25,so thefinal result is54.75whichbecomes thevalueofx.