David Cournapeau wrote: > bar += 1 > print bar is foo > > prints True > > But if I do bar = bar + 1, then bar is not a copy of foo anymore. Is > this intended ?
Yes, very much so! If it were otherwise Guido would never have added the augmented assignment operators. From the reference manual: http://docs.python.org/ref/augassign.html """An augmented assignment expression like x += 1 can be rewritten as x = x + 1 to achieve a similar, but not exactly equal effect. In the augmented version, x is only evaluated once. Also, when possible, the actual operation is performed in-place, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead.""" > This looks really confusing to me, and I would like to > know what the precise rules about copy vs alias are ? I'm not going to go through all of the functions in numpy, but here are some (unorganized) rules of thumb: * Any operations with regular operators + - / * // ** ~ ^ & | << >> < > == <=, etc. will put their results into new arrays. If the expression is on the RHS of a regular assignment, just ignore the LHS of the assignment; even if the name on the LHS is the same as a name on the RHS, the expression is evaluated, the new array is created and *then* the new array is assigned to the LHS name. * Augmented assignment operators will operate in-place. * Functions and methods should state that they operate in-place in their docstrings. * The asarray(), asanyarray(), etc. functions will return the input array if it is already suitable. * Slicing, reshaping, transposing and similar structural operations create new array objects, but they are generally views, not copies of the data. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco ------------------------------------------------------------------------- Using Tomcat but need to do more? Need to support web services, security? Get stuff done quickly with pre-integrated technology to make your job easier Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642 _______________________________________________ Numpy-discussion mailing list Numpy-discussion@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/numpy-discussion
