长话短说，我的Gist。

给定一个没有requirements.txt的Python项目，如果想知道需要安装哪些包才能满足这个项目的依赖需求，一个容易想到的方法就是对每一个.py文件，用模式匹配（如正则表达式）找import xxx，并记录xxx为需要的包。然而import语句有很多形式，如：import xxx、import xxx as aaa、import xxx as aaa, yyy as bbb、from xxx.yyy import fff as ccc、from .zzz import ggg。因此，更好的方法是利用抽象语法树ast模块来找出所有import语句。

Python的import语句对应ast的两种节点：ast.Import和ast.ImportFrom。要从ast.Import获取导入包的列表，可用：

[a.name for a in node.names]  # 其中node是ast.Import类型的

要从ast.ImportFrom获取导入的包，可用：

node.module  # 其中node是ast.ImportFrom类型的

值得注意的是如果当前import语句是from . import xxx，node.module将会是None，此时node.level > 0，意味着相对导入。因此，要想获得所有导入的包（除了相对导入外，因为相对导入的包绝不会是需要安装的依赖），可以这样：

import ast
# 假设source包含待解析源码
root = ast.parse(source)
result = []
for node in ast.walk(root):
    if isinstance(node, ast.Import):
        for a in node.names:
            result.append(a.name.split('.', maxsplit=1)[0])
    elif isinstance(node, ast.ImportFrom):
        if node.level == 0:
            result.append(node.module.split('.', maxsplit=1)[0])

然而绝对导入的包也有可能是工作目录中已存在的模块或包啊，此时我们就可以根据导入路径判断它是不是指工作目录下的包：

def exists_local(path, rootpkg):
    filepath = os.path.join(rootpkg, path.replace('.', os.path.sep))
    # see if path is a local package
    if os.path.isdir(filepath) and os.path.isfile(
            os.path.join(filepath, '__init__.py')):
        return True
    # see if path is a local module
    if os.path.isfile(filepath + '.py'):
        return True

    return False

其中path是导入路径，rootpkg是根包所在目录（定义见这里）。

把这个核心功能稍作包装，便可写出下面的完整可执行代码：

import argparse
import os
import ast
import sys
import pkgutil
import itertools


def make_parser():
    parser = argparse.ArgumentParser(
        description=('List all root imports. The *root* import of '
                     '`import pkg1.mod1` is "pkg1".'))
    parser.add_argument(
        '-g',
        '--greedy',
        action='store_true',
        help=('find also import statements within try block, '
              'if block, while block, function definition, '
              'etc.'))
    parser.add_argument(
        '-i',
        '--include-installed',
        action='store_true',
        help='include installed/built-in modules/packages')
    parser.add_argument(
        '--rootpkg',
        metavar='DIR',
        help=
        ('the directory of the root package. See '
         'https://docs.python.org/3.7/distutils/setupscript.html#listing-whole-packages '
         'about *root package*. If supplied, local packages/modules will be '
         'excluded from the results. For example, if '
         'there are "mod1.py" and "mod2.py", and in '
         '"mod2.py" there is `import mod1`, then "mod1" '
         'won\'t be listed in the result. Without it, '
         'only relative imports are excluded'))
    parser.add_argument(
        'filenames',
        metavar='SOURCE_FILE',
        nargs='*',
        help=('if specified multiple SOURCE_FILEs, the '
              'results will be joined; if specified no '
              'SOURCE_FILE, the stdin will be read for the '
              'source'))
    return parser


def exists_local(path, rootpkg):
    filepath = os.path.join(rootpkg, path.replace('.', os.path.sep))
    # see if path is a local package
    if os.path.isdir(filepath) and os.path.isfile(
            os.path.join(filepath, '__init__.py')):
        return True
    # see if path is a local module
    if os.path.isfile(filepath + '.py'):
        return True

    return False


# Reference: https://stackoverflow.com/a/9049549/7881370
def yield_imports(root, greedy: bool, rootpkg):
    traverse = ast.walk if greedy else ast.iter_child_nodes
    for node in traverse(root):
        if isinstance(node, ast.Import):
            for a in node.names:
                if (rootpkg and not exists_local(a.name, rootpkg)) \
                        or not rootpkg:
                    yield a.name.split('.', maxsplit=1)[0]
        elif isinstance(node, ast.ImportFrom):
            # if node.level > 0, the import is relative
            if node.level == 0 and (
                    (rootpkg and not exists_local(node.module, rootpkg)) \
                    or not rootpkg):
                yield node.module.split('.', maxsplit=1)[0]


def collect_results(filenames, greedy: bool, include_installed: bool, rootpkg):
    agg = []
    if not filenames:
        try:
            root = ast.parse(sys.stdin.read())
        except KeyboardInterrupt:
            pass
        else:
            agg.append(yield_imports(root, greedy, rootpkg))
    else:
        for filename in filenames:
            with open(filename) as infile:
                root = ast.parse(infile.read(), filename)
                agg.append(yield_imports(root, greedy, rootpkg))
    agg = set(itertools.chain.from_iterable(agg))
    if not include_installed:
        installed = set(
            itertools.chain(sys.builtin_module_names,
                            (x[1] for x in pkgutil.iter_modules())))
        agg = filter(lambda x: x not in installed, agg)
    agg = sorted(agg)
    return agg


def main():
    args = make_parser().parse_args()
    agg = collect_results(args.filenames, args.greedy, args.include_installed,
                          args.rootpkg)
    if agg:
        print('\n'.join(agg))


if __name__ == '__main__':
    main()

需要注意的是，程序的输出并不一定是PyPI上包的名字（例如，import bs4然而pip install beautifulsoup4）。