Python天文学计算入门

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"Python for Astronomers 是一本针对天文学科技工作者的入门教程，它介绍了如何使用Python进行科学计算和模拟。课程涵盖了基础的Unix技能、Python编程基础以及使用库和编写基本脚本等内容，旨在帮助天文学家利用Python进行数据处理和数值模拟。" 在天文学领域，Python因其强大的数据处理能力、丰富的库支持和易读性，已经成为了一个不可或缺的工具。本书首先介绍了基础的Unix技能，这对于在各种操作系统环境下工作非常重要。Unix是许多服务器和科研计算机的基础系统，掌握其基本操作如终端使用、SSH连接、常用命令（如改变目录、查看文件、创建和删除文件夹、移动/复制文件）能提升工作效率。接着，书中深入讲解了Python编程基础，包括数据类型（如整型、浮点型、字符串）、基本数学运算、变量的使用、字符串连接、数组、字符串和列表的索引以及切片操作。特别是二维切片，这对于处理多维数据，如天文图像或矩阵，十分关键。此外，还介绍了如何修改列表和数组，这对于数据处理和分析至关重要。然后，课程进入Python的库和基本脚本写作部分。Python有众多库支持各种任务，如NumPy用于数值计算，Astropy专门处理天文数据，Matplotlib用于数据可视化。了解如何导入和使用这些库是学习Python进行科学计算的关键步骤。编写脚本则是将单个命令组织成可重复使用的代码块，便于管理和复用。本书的作者Imad Pasha和Christopher Agostino来自加州大学伯克利分校的天文学系，他们感谢Pauline Arriage和Baylee Bordwell对课程内容的贡献，这些工作对于构建基础话题和教材产生了积极影响。本书的第一版发布于2015年，为天文学家提供了宝贵的Python学习资源，使他们能够更好地利用Python进行科学研究。

10 Chapter 1. Essential Unix Skills

UNIX system. In the course of this textbook, we will frequently want to indicate commands to by

typed into the terminal. Our format for doing so will be to represent the prompt with “»”, and to

usually indent commands on separate lines: for example, to exit a terminal, type

»exit

and the shell will close. Additionally, in this section of the text it will be useful to deﬁne a “typical”

series of nested directories, so that as we practice navigation, we can use self consistent examples.

For now lets use this typical tree: root/physics/user/sally/documents/homework/python/week1/

1.5.1 Changing Directories

There is a single command by which one can navigate the entire UNIX directory tree of any system,

and as there are several subtleties to it, we will discuss it in some detail. The command in question

is “cd”. The syntax “cd” is interpreted by the computer to mean “change directories”. Clearly

though, with just this command, it would be impossible for the computer to know where to change

directories to. Because of this, the command cd takes what is called an

argument

. An argument is a

part of the command necessary for it to function, but that is variable- the user can specify different

values for the argument within a certain set of possibilities. In this example, the “cd” command

takes as an argument a path location, for example in the command, // » cd /root/physics/sally //

/root/physics/sally serves as the argument to “cd”, it tells cd where to actually change directories to.

The majority of commands in UNIX have arguments, although there are a few exceptions.

So the question is, how can we efﬁciently use the cd command to navigate between directories in

UNIX? Luckily, the cd command has several built-in shortcuts that make navigating easier. However,

we would like to point out that these shortcuts only work in certain situations, which will be described

below. On the other hand, there is one sureﬁre argument for cd which will always work; unfortunately

it is the most cumbersome. Below, we describe the various ways to phrase arguments for cd, the ﬁrst

being the sure ﬁre method.

1. The full path

: From any directory in a UNIX system, typing cd followed by a full path,

starting with the root directory, will take you immediately to the speciﬁed location. This is

possible because a full path is unique, and thus the computer knows exactly where you mean

to go.

2. A nested directory

: For example, if you are in the directory “/root/physics/sally/” and want

to cd into the homework directory, you can simply type

»cd homework

This may seem confusing at ﬁrst, because there is no “/” before homework. Essentially, the

computer is interpreting your lack of a “/” to mean that the directory you are looking to cd into

is within the one you are currently in. (It will complain if you give a directory as an argument

that is not in the current one, if you use this syntax.

3. A secondary nested directory

: If you want to cd from a current location to two directories

deeper in the nesting system, you can start with the syntax from (2), but continue it into a

longer path, for example, if you were in “root/physics/sally/” and wanted to get into not just

“homework”, but all the way into “python” :

»cd homework/python

and further nested directories can be strung onto the end, if desired.

4. cd (space)

: defaults you to your username’s home directory. This will be different on

different systems, but on UGAstro it will take you to your username’s directory within the

home directory.

1.5 UNIX Commands 11

5. cd (space) .

: cd followed by a space and a period takes you to the current directory. In fact, a

single period is always a shortcut for “current directory,” in lots of different commands. Using

cd with just a period (as above) is pretty useless (it takes you nowhere). On the other hand:

»cd ..

is useful. This command will take you out one directory; i.e., if you were in the homework

directory of the sample tree, “cd ..” would take you to the “sally” directory. This command

can be strung together as well:

»cd ../..

brings you out two directories, and so forth. The single dot is most useful in other functions,

for example, when copying or moving ﬁles from remote directories to the directory you are in.

1.5.2 Viewing Files and Directories

You may notice that changing into a directory may change your prompt to reﬂect it, and/or typing

“pwd” will “print working directory” and show you where you are. (As a side note, we type “print”

though we arent printing things to a printer, because we are in essence printing the values from the

memory to the console screen). But none of this actually tells you what is in the directory in question,

unlike a GUI through which you actually see the ﬁles in a directory when you view it. No one’s

memory is perfect, so there is of course a command for viewing the contents of a directory. Typing:

»ls

into the terminal will print a list of the ﬁles and folders in the current directory in which you are

operating. ‘ls’ has many useful ﬂags for various situations, which are listed in the appendix under

the UNIX guide. Additionally, typing “»man(ls)” will bring up the manual for it (or any command

you choose), right in the terminal. Note that ls is one of those few commands that does not take an

explicit argument.

1.5.3 Making Directories

Now that we know how to view the contents in our directories, it becomes important for us to know

how to create and delete ﬁles and directories as well. In order to create a directory, use the command,

»mkdir desired_name

which you may notice is a shortened version of the phrase “make directory”. For instance, if one

wants to create a directory in which to store their speciﬁc information regarding this course, one

would type into the terminal ‘mkdir python_decal’. If we use ‘ls’ we will see that this new directory

is included in the contents listed. It is imperative to note that you should avoid using spaces when

using the mkdir command as placing a space between two or more words will just end up creating

directories named after each individual word. If you really must have a space in your directory,

you can type a backslash before the desired space as this tells UNIX to ignore the space. On

shared-network ﬁle systems, having an organized system of directories that make logical sense is

very helpful, both for keeping yourself organized and for allowing you to direct others to speciﬁc

ﬁles and folders more easily.

1.5.4 Deleting Files and Directories

Now that we know how to create directories, move between them, and look at the ﬁles inside, the

next step is to learn how to delete things.

NOTE: UNIX is not like a Windows or Mac where ﬁles are sent to a trash bin. When you hit

12 Chapter 1. Essential Unix Skills

delete, things are gone forever.

That being said, removing ﬁles is a relatively easy task in UNIX. If you are in the directory where

the ﬁle to be deleted is stored, simply type

»rm ﬁlename

to delete it. (Note: ug astro servers have a setting turned on such that it will ask you if you are sure

you want to delete. This is not native to the rm command, and on another system, do not expect rm

to query you). Removing directories requires the use of a

ﬂag

, or

option

. A ﬂag/option is a modiﬁer

added after a command, before the argument, that changes exactly how the command is carried out.

for example, typing “ls -a” will list “all” ﬁles, whereas “ls” alone usually ignores hidden ﬁles and

directories. In this case, we need to make use of the “recursive” option:

»rm -r directory_name

which will go into a directory, delete the ﬁles within, and then delete the directory itself. The example

here illustrates the syntax for using ﬂags/options in general (with a dash preceding the ﬂag).

There is a secondary way of deleting directories, which may be easier to remember:

»rmdir directory_name which will also delete the directory in question and everything within it.

1.5.5 Moving/Copying Files and Directories

The last major skill needed for operating in UNIX ﬁle systems is moving and copying ﬁles and

directories from one place to another. Moving is done using the “move” command (funnily enough):

»mv ﬁlename new_location

(this assumes you are in the directory with the ﬁle to be moved. Depending on where you are moving

the ﬁle to, the new_location could be as simple as “..” or as complex as a full pathname to another

directory tree.).

Mv also gives you the option of changing the name of a ﬁle as you move it, for example:

»mv ﬁle_name new_name new_location

would move “ﬁle_name” to “new_location”, changing its name to “new_name” along the way.

Interestingly, because of this functionality, mv serves as the “rename” command as well. To rename

a ﬁle, “move” it to a new name without specifying a new location to send it. If you want to copy a

ﬁle instead of moving it, use:

»cp ﬁlename new_location

which will create a copy and put it in “new_location”. Cp also has the ability to rename ﬁles in

transit, by the same syntax as mv.

1.5.6 The Wildcard

One extremely useful thing to know about UNIX is the ability to use wild cards. Denoted by a “*”

symbol, wildcards can stand for any character, or any number of characters. The strategic use of

wildcards can save you a lot of time when working with large numbers of ﬁles. A few examples

should make clear how wildcards are used:

1. Deleting many ﬁles

: Say for example you wanted to delete all ﬁles in a certain directory that

were of the type .doc (or docx for all you millenials). If you entered

»rm *.doc

The wildcard would feed rm every ﬁle with any combination of characters that ended in

.doc for deletion. In a similar vein, if you have a group of research ﬁles that all started

with “simulation_run1” (where an example ﬁlename might be simulation_run10004.dat,

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