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| -rw-r--r-- | Doc/ref/ref7.tex | 22 |
1 files changed, 20 insertions, 2 deletions
diff --git a/Doc/ref/ref7.tex b/Doc/ref/ref7.tex index cacd8ef..bfceeea 100644 --- a/Doc/ref/ref7.tex +++ b/Doc/ref/ref7.tex @@ -304,8 +304,7 @@ executed only when the function is called. When one or more top-level parameters have the form \var{parameter} \code{=} \var{expression}, the function is said to have ``default -parameter values.'' \strong{Default parameter values are evaluated -when the function definition is executed.} For a parameter with a +parameter values.'' For a parameter with a default value, the corresponding argument may be omitted from a call, in which case the parameter's default value is substituted. If a parameter has a default value, all following parameters must also have @@ -315,6 +314,25 @@ expressed by the grammar.% \code{def f(a=1, b)} is interpreted as \code{def f(a=1, b=None)}.} \indexiii{default}{parameter}{value} +\strong{Default parameter values are evaluated when the function +definition is executed.} This means that the expression is evaluated +once, when the function is defined, and that that same +``pre-computed'' value is used for each call. This is especially +important to understand when a default parameter is a mutable object, +such as a list or a dictionary: if the function modifies the object +(e.g. by appending an item to a list), the default value is in effect +modified. This is generally not what was intended. A way around this +is to use \code{None} as the default, and explicitly test for it in +the body of the function, e.g.: + +\begin{verbatim} +def whats_on_the_telly(penguin=None): + if penguin is None: + penguin = [] + penguin.append("property of the zoo") + return penguin +\end{verbatim} + Function call semantics are described in more detail in section \ref{calls}. A function call always assigns values to all parameters mentioned in |